When the ECM sets a Diagnostic Trouble Code (DTC), the Malfunction Indicator Lamp (MIL) lamp will be turned on only for type A, B and E but a DTC will be stored in the ECM's memory for all types of DTC. If the problem is intermittent, the MIL will go out after 10 seconds if the fault is no longer present. The DTC will stay in the ECM's memory until cleared by scan tool. Removing battery voltage for 10 seconds will clear some stored DTCs.

DTCs should be cleared after repairs have been completed. Some diagnostic tables will tell you to clear the codes before using the chart. This allows the ECM to set the DTC while going through the chart, which will help to find the cause of the problem more quickly.

The Engine Control Module (ECM) uses the Manifold Absolute Pressure (MAP) sensor to control the fuel delivery and the ignition timing. The MAP sensor measures the changes in the intake manifold pressure which results from engine load (intake manifold vacuum) and the rpm changes, and it converts these into voltage outputs. The ECM can detect if the MAP sensor is not responding to the Throttle Position (TP) changes by comparing the actual MAP change to a predicted MAP change based on the amount of TP change that occurs. If the ECM does not see the expected MAP change or more, DTC P0106 will set.

With the ignition ON and the engine stopped, the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the ECM as an indication of vehicle altitude. Comparison of this reading with a known good vehicle with the same sensor is a good way to check the accuracy of a suspect sensor. Readings should be the same +0.4 volts.

The MAP sensor vacuum source should be thoroughly checked for restrictions at the intake manifold.

The Manifold Absolute Pressure (MAP) sensor responds to changes in intake manifold pressure (vacuum). The MAP signal voltage to the Engine Control Module (ECM) varies from below 2 volts at idle (high vacuum) to above 4 volts with the key in the ON position, engine not running or at Wide Open Throttle (WOT) (low vacuum).

A "speed density" method of determining engine load is used. This is calculated using inputs from the MAP sensor, the rpm (58X), and the Intake Air Temperature (IAT) sensor. The MAP sensor is the main sensor used in this calculation, and measuring engine load is its main function.

The MAP sensor is also used to determine manifold pressure changes while the linear Exhaust Gas Recirculation (EGR) flow test diagnostic is being run (refer to DTC P0401). This determines the engine vacuum level for some other diagnostics and determines Barometric Pressure (BARO). The ECM compares the MAP sensor signal to calculated MAP based on Throttle Position (TP) and various other engine load factors. If the ECM detects a MAP signal voltage that is intermittently above the calculated value, DTC P1106 will set.

Check for the following conditions:

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

The Engine Control Module (ECM) uses the Manifold Absolute Pressure (MAP) sensor to control the fuel delivery and the ignition timing. The MAP sensor measures the changes in the intake manifold pressure which results from engine load (intake manifold vacuum) and the rpm changes, and it converts these into voltage outputs. The ECM sends a 5 volt reference voltage to the MAP sensor. As the manifold pressure changes, the output of MAP sensor also changes. By monitoring the Map sensor output voltage, the ECM knows the manifold pressure. A low pressure (low voltage) output voltage will be about 1.0 to 1.5 volts while the higher pressure (high voltage) output voltage will be about 4.5 to 4.8 volts at Wide Open Throttle(WOT). The MAP sensor is also used, under certain conditions to measure Barometric Pressure (BARO), allowing the ECM to make adjustments for different altitude.

With the ignition ON and the engine stopped, the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the ECM as an indication of vehicle altitude. Comparison of this reading with a known good vehicle with the same sensor is a good way to check the accuracy of a suspect sensor. Readings should be the same ±12 kPa.

If a DTC P0107 is intermittent, refer to "Manifold Absolute Pressure Check: in this section for further diagnosis.

The Manifold Absolute Pressure (MAP) sensor responds to changes in intake manifold pressure (vacuum). The MAP signal voltage to the Engine Control Module (ECM) varies from below 2 volts at idle (high vacuum) to above 4 volts with the key in the ON position, engine not running or at Wide Open Throttle (WOT) (low vacuum).

A "speed density" method of determining engine load is used. This is calculated using inputs from the MAP sensor, the rpm (58X), and the Intake Air Temperature (IAT) sensor. The MAP sensor is the main sensor used in this calculation, and measuring engine load is its main function.

The MAP sensor is also used to determine manifold pressure changes while the linear Exhaust Gas Recirculation (EGR) flow test diagnostic is being run (refer to DTC P0401). This determines the engine vacuum level for some other diagnostics and determines Barometric Pressure (BARO). The ECM compares the MAP sensor signal to calculated MAP based on Throttle Position (TP) and various other engine load factors. If the ECM detects a MAP signal voltage that is intermittently below the calculated value, DTC P1107 will set.

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

The Engine Control Module (ECM) uses the Manifold Absolute Pressure (MAP) sensor to control the fuel delivery and the ignition timing. The MAP sensor measures the changes in the intake manifold pressure which results from engine load (intake manifold vacuum) and the rpm changes, and it converts these into voltage outputs. The ECM sends a 5 volt reference voltage to the MAP sensor. As the manifold pressure changes, the output of MAP sensor also changes. By monitoring the Map sensor output voltage, the ECM knows the manifold pressure. A low pressure (low voltage) output voltage will be about 1.0 to 1.5 volts while the higher pressure (high voltage) output voltage will be about 4.5 to 4.8 volts at Wide Open Throttle(WOT). The MAP sensor is also used, under certain conditions to measure Barometric Pressure (BARO), allowing the ECM to make adjustments for different altitude.

With the ignition ON and the engine stopped, the manifold pressure is equal to atmospheric pressure and the signal voltage will be high. This information is used by the ECM as an indication of vehicle altitude. Comparison of this reading with a known good vehicle with the same sensor is a good way to check the accuracy of a suspect sensor. Readings should be the same ±12 kPa.

If a DTC P0108 is intermittent, refer to "Manifold Absolute Pressure Check" in this section for further diagnosis.

DTC P0108 may set as result of a misfire. If misfire is present, repair the cause of misfire before using this table. The misfire counters may be used to determine which cylinder(s) is misfiring.

If DTC P0172 is also set, check 5 volt reference circuit for short to voltage.

The Intake Air Temperature (IAT) sensor is a thermistor which measures the temperature of the air entering the engine. The Engine Control Module (ECM) applies 5 volts through a pull-up resistor to the IAT sensor. When the intake air is cold, the resistance is high, and the ECM will monitor a high signal voltage on the IAT signal circuit. If the intake air is warm, the sensor resistance is lower causing the ECM to monitor a lower voltage. Diagnostic Trouble Code (DTC) P1111 will set when the ECM detects an intermittent high signal voltage in the intake air temperature sensor signal circuit or sensor.

Check for the following conditions:

The Intake Air Temperature (IAT) sensor uses a thermistor to control the signal voltage to the Engine Control Module (ECM). The ECM supplies a 5 volt reference and a ground to the sensor. When the air is cold, the resistance is high; therefore the IAT signal voltage will be high. If the intake air is warm, resistance is low; therefore the IAT signal voltage will be low.

The Intake Air Temperature (IAT) sensor is a thermistor which measures the temperature of the air entering the engine. The Engine Control Module (ECM) applies 5 volts through a pull-up resistor to the IAT sensor. When the intake air is cold, the resistance is high, and the ECM will monitor a high signal voltage on the IAT signal circuit. If the intake air is warm, the sensor resistance is lower causing the ECM to monitor a lower voltage. Diagnostic Trouble Code (DTC) P1111 will set when the ECM detects an intermittent high signal voltage in the intake air temperature sensor signal circuit or sensor.

Check for the following conditions:

The Intake Air Temperature (IAT) sensor uses a thermistor to control the signal voltage to the Engine Control Module (ECM). The ECM supplies a 5 volt reference and a ground to the sensor. When the air is cold, the resistance is high; therefore the IAT signal voltage will be high. If the intake air is warm, resistance is low; therefore the IAT signal voltage will be low.

The Engine Coolant Temperature sensor (ECT) is a thermistor mounted in the engine coolant stream. The Engine control Module (ECM) applies 5 volt reference voltage through a pull-up resistor to the ECT signal circuit. When the engine coolant is cold, the sensor resistance is high, and the ECM will monitor a high signal voltage. As the engine coolant warms, the sensor resistance is less, and the ECT signal voltage measured at the ECM drops. With a fully warmed engine, the ECT signal voltage should measure about 1.5 to 2.0 volts. If the ECM detects an ECT signal that is intermittently below the range of the ECT sensor, Diagnostic Trouble Code (DTC) P1114 will set.

Check for the following conditions:

The Engine Coolant Temperature sensor (ECT) is a thermistor mounted in the engine coolant stream. The Engine control Module (ECM) applies 5 volt reference voltage through a pull-up resistor to the ECT signal circuit. When the engine coolant is cold, the sensor resistance is high, and the ECM will monitor a high signal voltage. As the engine coolant warms, the sensor resistance is less, and the ECT signal voltage measured at the ECM drops. With a fully warmed engine, the ECT signal voltage should measure about 1.5 to 2.0 volts. If the ECM detects an ECT signal that is intermittently above the range of the ECT sensor, Diagnostic Trouble Code (DTC) P1115 will set.

Check for the following conditions:

The Engine Coolant Temperature (ECT) sensor uses a thermistor to control the signal voltage to the Engine Control Module (ECM). The ECM supplies a 5 volt reference and a ground to the sensor. When the engine coolant is cold, the resistance is high; therefore the ECT signal voltage will be high. If the engine coolant is warm, sensor resistance becomes less; therefore the IAT signal voltage drops. At normal engine operating temperature, the voltage will be between 1.5 to 2.0 volts at the ECT signal terminal.

The ECT sensor is used to control following items:

The Engine Coolant Temperature (ECT) sensor uses a thermistor to control the signal voltage to the Engine Control Module (ECM). The ECM supplies a 5 volt reference and a ground to the sensor. When the engine coolant is cold, the resistance is high; therefore the ECT signal voltage will be high. If the engine coolant is warm, sensor resistance becomes less; therefore the IAT signal voltage drops. At normal engine operating temperature, the voltage will be between 1.5 to 2.0 volts at the ECT signal terminal.

The ECT sensor is used to control following items:

The Throttle Position (TP) sensor circuit provides a voltage signal that changes relative to throttle blade angle. The TP sensor sends a voltage signal back to the Engine Control Module (ECM) relative to the throttle plate opening. The voltage signal will vary from approximately 0.33 volts at closed throttle, to over 4.3 volts at Wide Open Throttle (WOT).

The TP signal is used by the ECM for fuel control and for most of the ECM controlled outputs. The TP signal is one of the most important inputs used by the ECM for fuel control and most of the ECM controlled outputs. If the ECM detects a TP signal that is intermittently above the range of the TP sensor, Diagnostic Trouble Code (DTC) P1121 will be set.

Check for the following conditions:

If DTC P1121 cannot be duplicated, reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

The Engine Control Module (ECM) supplies a 5 volt reference voltage signal and a ground to the Throttle Position (TP) sensor. The TP sensor sends a voltage signal back to the ECM relative to the throttle plate opening. The voltage signal will vary from approximately 0.33 volts at closed throttle, to over 4.3 volts at Wide Open Throttle (WOT).

The TP signal is used by the ECM for fuel control and for most of the ECM controlled outputs. The TP signal is one of the most important inputs used by the ECM for fuel control and most of the ECM controlled outputs.

If the DTC P0122 cannot be duplicated, the information included in the Freeze Frame data can be useful. Use a scan tool information data to determine the status of the DTC. If the dc occurs intermittently, using the Diagnostic table may help isolate the problem.

The Throttle Position (TP) sensor circuit provides a voltage signal that changes relative to throttle blade angle. The TP sensor sends a voltage signal back to the Engine Control Module (ECM) relative to the throttle plate opening. The voltage signal will vary from approximately 0.33 volts at closed throttle, to over 4.3 volts at Wide Open Throttle (WOT).

The TP signal is used by the ECM for fuel control and for most of the ECM controlled outputs. The TP signal is one of the most important inputs used by the ECM for fuel control and most of the ECM controlled outputs. If the ECM detects a TP signal that is intermittently above the range of the TP sensor, Diagnostic Trouble Code (DTC) P1122 will be set.

Check for the following conditions:

If DTC P1122 cannot be duplicated, reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

The Engine Control Module (ECM) supplies a 5 volt reference voltage signal and a ground to the Throttle Position (TP) sensor. The TP sensor sends a voltage signal back to the ECM relative to the throttle plate opening. The voltage signal will vary from approximately 0.33 volts at closed throttle, to over 4.3 volts at Wide Open Throttle (WOT).

The TP signal is used by the ECM for fuel control and for most of the ECM controlled outputs. The TP signal is one of the most important inputs used by the ECM for fuel control and most of the ECM controlled outputs.

If the DTC P0123 cannot be duplicated, the information included in the Freeze Frame data can be useful. Use a scan tool information data to determine the status of the DTC. If the dc occurs intermittently, using the Diagnostic table may help isolate the problem.

With ignition ON and the throttle at closed position, the voltage should read between 0.2 and 0.90 volts and increase steadily to over 4.3 volts at WOT.

DTCs P0123 and P0113 stored at the same time could be result of an open sensor ground circuit.

When the vehicle is first started, it operates in Open Loop, ignoring the oxygen sensor (O2S) signal and calculating the fuel/air ratio based on inputs from the Engine Coolant Temperature (ECT), Throttle Position (TP), and Manifold Absolute Pressure (MAP) sensors only. The Engine Control Module (ECM) will begin using the O2S signal for controlling fuel delivery (Closed Loop) when the following conditions are met:

When DTC P0125 is set, a skewed ECT sensor or a stuck open thermostat is indicated.

An intermittent may be caused by a poor connection, rubbed-through wire insulation or a wire broken inside the insulation.

Check for a poor connection or damaged ECM harness. Inspect the ECT sensor signal circuit and ground circuit terminals for the following conditions:

Perform an intermittent test. If the connections and the harness check OK, monitor a Digital Voltmeter connected between ECT sensor signal circuit and ground circuit terminals while moving the related connectors and the wiring harness. If a fault is induced, the resistance reading will change. This may help to isolate the location of the malfunction.

Use the Temperature vs. Resistance table to evaluate the possibility of a skewed sensor.

The Engine Control Module (ECM) supplies a voltage of about 0.45 volts between terminals M12 and M29 (if measured with a 10 megohm digital voltmeter, this may read as low as 0.32 volts). The Oxygen Sensor (O2S) varies the voltage within a range of about 1 volt if the exhaust is rich, down through about 0.10 volts if the exhaust is lean.

The sensor is like an open circuit and produces no voltage when it is below 315°C (600°F). An open sensor circuit or cold sensor causes Open Loop operation.

If the O2S pigtail wiring, connector, or terminal is damaged, the entire O2S assembly must be replaced. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, it must have a clean air reference provided to it. This clean air reference is obtained by way of the O2S wire(s). Any attempt to repair the wires, connector or terminals could result in the obstruction of the air reference and degrade the O2S performance. Refer to "Oxygen Sensor" in this section.

The DTC P0131 or lean exhaust is most likely caused by one of the following items:

The Engine Control Module (ECM) supplies a voltage of about 0.45 volts between terminals M12 and M29 (if measured with a 10 megohm digital voltmeter, this may read as low as 0.32 volts). The Oxygen Sensor (O2S) varies the voltage within a range of about 1 volt if the exhaust is rich, down through about 0.10 volts if the exhaust is lean.

The sensor is like an open circuit and produces no voltage when it is below 315°C (600°F). An open sensor circuit or cold sensor causes Open Loop operation.

If the O2S pigtail wiring, connector, or terminal is damaged, the entire O2S assembly must be replaced. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, it must have a clean air reference provided to it. This clean air reference is obtained by way of the O2S wire(s). Any attempt to repair the wires, connector or terminals could result in the obstruction of the air reference and degrade the O2S performance. Refer to "Oxygen Sensor" in this section.

The DTC P0132 or rich exhaust is most likely caused by one of the following items:

The Engine Control Module (ECM) continuously monitors the Oxygen Sensor (O2S) activity for 100 seconds. During the monitor period, the ECM counts the number of times that the O2S switches from rich to lean and from lean to rich and adds the amount of time it took to complete all switches. With this information, an average time for all switches can be determined. If the average time to switch is too out of specification, a DTC P0133 will set.

If the O2S pigtail wiring, connector, or terminal is damaged, the entire O2S assembly must be replaced. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, it must have a clean air reference provided to it. This clean air reference is obtained by way of the O2S wire(s). Any attempt to repair the wires, connector, or terminals could result in the obstruction of the air reference and degrade O2S performance.

DTC P0133 or slow response is most likely caused by one of the following items:

The Engine Control Module (ECM) continuously monitors the Oxygen Sensor (O2S) activity for 100 seconds. During the monitor period, the ECM counts the number of times that the O2S switches from rich to lean and from lean to rich. With this information, a total for all switches can be determined. If the number of switches is too low, a Diagnostic Trouble code (DTC) P1133 will set. The lean-to-rich and rich-to-lean are less than 15 switches.

DTC P1133 is most likely caused by one of the following items:

The Engine Control Module (ECM) supplies a voltage of about 0.45 volts between terminals M12 and M29 (if measured with a 10 megohm digital voltmeter, this may read as low as 0.32 volts). The Oxygen Sensor (O2S) varies the voltage within a range of about 1 volt if the exhaust is rich, down through about 0.10 volts if the exhaust is lean.

The sensor is like an open circuit and produces no voltage when it is below 315°C (600°F). An open sensor circuit or cold sensor causes Open Loop operation.

If the O2S pigtail wiring, connector, or terminal is damaged, the entire O2S assembly must be replaced. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, it must have a clean air reference provided to it. This clean air reference is obtained by way of the O2S wire(s). Any attempt to repair the wires, connector or terminals could result in the obstruction of the air reference and degrade the O2S performance. Refer to "Oxygen Sensor" in this section.

The Engine Control Module (ECM) monitors the Oxygen Sensor (O2S) activity for 100 seconds after closed loop and stoichiometric operation have been established. During the monitoring period the ECM counts the number of times that the O2S responds from rich to lean and adds the amount of time it took to complete all transitions. With this information, an average time for all transitions can be determined. The ECM then divides the -to-lean average by the lean-to-rich average to obtain the ratio. If the O2S transition time ratio is not within the range, Diagnostic Trouble Code (DTC) P1134 will be set, indicating that the O2S is not responding as expected to changes in exhaust oxygen content.

In order to control emissions, a catalytic converter is used to convert harmful emissions into harmless water vapor and carbon dioxide.

The Engine Control Module (ECM) has the ability to monitor this process by using a Heated Oxygen Sensor (HO2S). The HO2S, located in the exhaust stream past the catalytic converter, produces an output signal which indicates the storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert exhaust emissions effectively. If the catalyst is functioning properly, the HO2S signal will be far less active than the signal produced by the Oxygen Sensor (O2S).

If the HO2S pigtail wiring, connector, or terminal is damaged, the entire HO2S assembly must be replaced. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, it must have a clean air reference provided to it. This clean air reference is obtained by way of the HO2S wire(s). Any attempt to repair the wires, connector, or terminals could result in the obstruction of the air reference and degrade the HO2S performance.

An intermittent may be caused by rubbed-through wire insulation or a wire contacting the exhaust.

Check for the following conditions:

In order to control emissions, a catalytic converter is used to convert harmful emissions into harmless water vapor and carbon dioxide.

The Engine Control Module (ECM) has the ability to monitor this process by using a Heated Oxygen Sensor (HO2S). The HO2S, located in the exhaust stream past the catalytic converter, produces an output signal which indicates the storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert exhaust emissions effectively. If the catalyst is functioning properly, the HO2S signal will be far less active than the signal produced by the Oxygen Sensor (O2S).

If the HO2S pigtail wiring, connector, or terminal is damaged, the entire HO2S assembly must be replaced. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, it must have a clean air reference provided to it. This clean air reference is obtained by way of the HO2S wire(s). Any attempt to repair the wires, connector, or terminals could result in the obstruction of the air reference and degrade the HO2S performance.

Check for the following conditions:

In order to control emissions, a catalytic converter is used to convert harmful emissions into harmless water vapor and carbon dioxide.

The Engine Control Module (ECM) has the ability to monitor this process by using a Heated Oxygen Sensor (HO2S). The HO2S, located in the exhaust stream past the catalytic converter, produces an output signal which indicates the storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert exhaust emissions effectively. If the catalyst is functioning properly, the HO2S signal will be far less active than the signal produced by the Oxygen Sensor (O2S).

If the HO2S pigtail wiring, connector, or terminal is damaged, the entire HO2S assembly must be replaced. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, it must have a clean air reference provided to it. This clean air reference is obtained by way of the HO2S wire(s). Any attempt to repair the wires, connector, or terminals could result in the obstruction of the air reference and degrade the HO2S performance.

An intermittent may be caused by a rubbed-through wire insulation or a wire contacting the exhaust.

Check for the following conditions:

In order to control emissions, a catalytic converter is used to convert harmful emissions into harmless water vapor and carbon dioxide.

The Engine Control Module (ECM) has the ability to monitor this process by using a Heated Oxygen Sensor (HO2S). The HO2S, located in the exhaust stream past the catalytic converter, produces an output signal which indicates the storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert exhaust emissions effectively. If the catalyst is functioning properly, the HO2S signal will be far less active than the signal produced by the Oxygen Sensor (O2S).

If the HO2S pigtail wiring, connector, or terminal is damaged, the entire HO2S assembly must be replaced. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, it must have a clean air reference provided to it. This clean air reference is obtained by way of the HO2S wire(s). Any attempt to repair the wires, connector, or terminals could result in the obstruction of the air reference and degrade the HO2S performance.

The ECM will check if HO2S is functioning properly by monitoring the current to HO2S heater. If the current is less than 0.1 ampere, DTC P0141 will set.

An intermittent may be caused by a rubbed-through wire insulation or a wire contacting the exhaust.

Check for a poor connection or a damaged harness -inspect the harness connectors for the following conditions:

The Engine Control Module (ECM) supplies a voltage of about 0.45 volts between terminals M12 and M29 (if measured with a 10 megohm digital voltmeter, this may read as low as 0.32 volts). The Oxygen Sensor (O2S) varies the voltage within a range of about 1 volt if the exhaust is rich, down through about 0.10 volts if the exhaust is lean.

In internal circuitry of the Engine control Module (ECM) can identify if the vehicle fuel system is capable of cutoff amount of the fuel supply during deceleration. When a Decel Fuel Cutoff (DFCO) mode of operation is requested during Closed Loop operation, the ECM will cutoff the fuel supply to the engine. Under these conditions the ECM should detect a lean condition. If the ECM detect a rich condition at this time, Diagnostic Trouble Code (DTC) P1167 will set. Damaged fuel pressure regulator and faulty injector will be the cause of this DTC.

The DTC P1167 or rich exhaust is most likely caused by one of the following items:

To provide the best possible combination of driveability, fuel economy, and emission control, a Closed Loop air/ fuel metering system is used. While in Closed Loop, the Engine Control Module (ECM) monitors the oxygen sensor (O2S) signal voltage and adjusts fuel delivery based on signal voltage. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with the scan tool. Ideal fuel trim values are around 128 (0%). If the O2S signal is indicating a lean condition, the ECM will add fuel resulting in fuel trim values above 128 (0% to 100%). If a rich condition is detected, the fuel trim values will be below 128 (0% to −100%), indicating that the ECM is reducing the amount of fuel delivered. If exhaust emissions reach an excessive level due to a lean or rich condition, a fuel trim Diagnostic Trouble Code (DTC) is set.

Check for poor O2S or MAP sensor connection at the ECM. Inspect the harness connectors for the following conditions:

Inspect the wiring harness for damage. If the harness appears to be OK, observe the O2S display on the scan tool while moving the connectors and the wiring harness related to the engine harness. A change in the display will indicate the location of the fault.

Check the brake power booster check valve for possible leaks.