For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptor

DTC P0171 00: Fuel Trim System Lean
DTC P0172 00: Fuel Trim System Rich

### Circuit/System Description

The engine control module (ECM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. Fuel delivery is controlled differently during open loop and closed loop. During open loop, the ECM determines fuel delivery based on sensor signals without heated oxygen sensor (HO2S) input. During closed loop, the ECM adds HO2S inputs and level of purge to calculate the short and long term fuel trim adjustments. If the HO2S indicates a lean condition, the fuel trim values will be above 0%. If the HO2S indicates a rich condition, the fuel trim values will be below 0%. The short term fuel trim values change rapidly in response to the HO2S voltage signals. The long term fuel trim makes rough adjustments in order to maintain an optimal air/fuel ratio. A block of memory cells contain information arranged in combinations of engine RPM and engine load for a full range of vehicle operating conditions. The long term fuel trim diagnostic is based on an average of cells currently being used. The ECM selects the cells based on engine speed and load. The fuel trim diagnostic will conduct a test to determine if a rich failure actually exists or if excessive vapor from the evaporative emission (EVAP) canister is causing a rich condition.

The heating elements inside each heated oxygen sensor (HO2S) heat the sensor to bring the sensor up to operating conditions faster. This allows the system to enter closed loop earlier and the control module to calculate the air-to-fuel ratio sooner.

The heated oxygen sensor (HO2S) utilizes the following circuits:

• | A signal circuit

• | A low reference circuit

• | An ignition voltage circuit

• | A heater control circuit

### Conditions for Running the DTC

#### P0171 00

• | DTCs P0101 00, P0102 00, P0103 00, P0106 00, P0107 00, P0108 00, P0112 00, P0113 00, P0201 00, P0202 00, P0203 00, P0204 00, P0300 00, P0301 00, P0302 00, P0303 00, P0304 00, P0442 00, P0443 00, P0449 00, P0451 00, P0452 00, P0453 00, P0454 00, P0455 00, P0461 00, P0462 00, P0463 00, P0464 00, P0496 00, P0506 00, and P0507 00 are not set.

• | Where applicable DTCs P2270 00, P2271 00, P2272 00, P2A01 00, are not set.

• | The loop status parameter indicates closed.

• | The fuel trim learn is enabled.

• | The engine coolant temperature (ECT) is between -38 and +1530°C (-36.4°F and +266°F).

• | The intake air temperature (IAT) is between -38 and +150°C (-36.4°F and +302°F).

• | The manifold absolute pressure (MAP) is between 15-255 kPa (2-37 psi).

• | The vehicle speed is less than 134 km/h (83 mph).

• | The engine speed is between 400-6000 RPM.

• | The engine airflow is between 1-512 g/s.

• | The fuel level is greater than 10%.

• | The barometric pressure (BARO) is more than 70 kPa.

• | DTCs P0171 00 and P0172 00 run continuously when the above conditions have been met.

### Conditions for Setting the DTC

• | The average long term weighted average value is more or less than a calibrated value.

• | The above condition is present for approximately 3 min after the conditions for running the DTC have been met.

### Action Taken When DTC Sets

DTCs P017 001, P0172 00 are type B DTCs.

### Conditions for Clearing the DTC

DTCs P0171 00, P0172 00 are type B DTCs.

### Diagnostic Aids

• | On vehicles equipped with a vacuum brake booster, rapid brake pedal cycling may imitate a vacuum leak.

• | The system will become lean if a fuel injector is not supplying enough fuel - DTC P0171 00

• | A lean condition may be present during high fuel demand due to a fuel pump that does not deliver enough fuel, a plugged fuel filter, or a restricted fuel pipe - DTC P017 001.

• | The system will become rich if a fuel injector is supplying too much fuel - DTC P0172 00.

• | An exhaust system leak that may not be audible but can cause a DTC to set.

• | A MAF sensor that is installed incorrectly can set this these DTC's.

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### Connector End View Reference

Component Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

Note: Disregard any transmission symptoms, antilock brake system (ABS) indicators, and traction control system (TCS) indicators until any fuel trim faults are repaired. A fuel trim fault may cause default actions such as harsh shifts and illumination of the ABS/TCS indicators.

1. Verify that other DTCs are not set.

⇒ | If any DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle

2. Allow the engine to reach operating temperature. With the engine running, observe the B52A Heated Oxygen Sensor (HO2S) parameter with a scan tool. The B52A Heated Oxygen Sensor (HO2S) value should vary from approximately 40 mV to approximately 900 mV, and respond to fueling changes.

⇒ | If the value does not vary from approximately 40 mV to approximately 900 mV, refer to DTC P0133, P0134, P013A, P013B, P013E, P013F, P0140, P1133, P2270, P2271, or P2A00

Note: The EVAP purge enablement may cause the fuel trim to be momentarily outside the normal range

3. The normal short term fuel trim parameter should be between +5% to -5%, with near 0% the optimum, with the engine running at operating temperature.
4. The long term fuel trim parameter should be between +5% to -5%, with near 0% the optimum, with the engine running at operating temperature.
5. Operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that you observed from the freeze frame/failure records data.

### Circuit/System Testing

#### P0171 00

Allow the engine to reach operating temperature. With the engine running, observe the Long Term Fuel Trim parameter with a scan tool. The value should be between +5% to -5%.

⇒ | If not within the specified range, verify none of the following conditions exist

• | Determine the current vehicle testing altitude. Compare the MAP Sensor pressure parameter to the reading in the Need link for Altitude Versus Barometric Pressure

⇒ | If the B74 Manifold Absolute Pressure (MAP) Sensor is not within the range specified for your altitude, refer to DTC P0106

• | With the engine idling, observe the MAF sensor parameter. The MAF sensor parameter should be between 2 to 6 g/s at idle.

⇒ | If the MAF sensor parameter is not between 2 to 6 g/s at idle, refer to need link for (DTC P0101)

• | Vacuum hoses for splits, kinks, and improper connections.

• | Insufficient fuel in the tank.

• | Low fuel pressure, refer to Fuel System Diagnosis

• | Ethanol concentration greater than 15%, refer to Alcohol/Contaminants-in-Fuel Diagnosis

• | Fuel contamination, refer to Alcohol/Contaminants-in-Fuel Diagnosis

• | Malfunctioning fuel injectors, refer to Fuel Injector Diagnosis

• | Missing, loose, or leaking exhaust components from the B52A Heated Oxygen Sensor (HO2S) forward, refer to Symptoms - Engine Exhaust

• | Vacuum leaks at the intake manifold, the Q38 Throttle Body, and the Q17 Fuel Injector O-rings.

• | The air induction system and the air intake ducts for leaks or for a missing air filter element.

• | A cracked EVAP canister.

• | Evaporative pipes obstructed or leaking.

• | The crankcase ventilation system for leaks, refer to Crankcase Ventilation System Inspection/Diagnosis

• | The B52A Heated Oxygen Sensor (HO2S) for improper installation and for electrical wires or connectors that may have contacted the exhaust system.

• | The B52A Heated Oxygen Sensor (HO2S) signal circuit open, shorted to ground, or shorted to the low reference circuit.

• | Malfunctioning engine components, refer to Symptoms - Engine Mechanical

#### P0172 00

Allow the engine to reach operating temperature. With the engine running, observe the Long Term Fuel Trim parameter with a scan tool. The reading should be between +5% to -5%.

⇒ | If not within the specified range, inspect for the following:

• | With the engine idling and the transmission in the park or neutral position, observe the MAP Sensor parameter. The MAP Sensor parameter should be between 19 to 42 kPa.

⇒ | If the MAP Sensor parameter is not between 19 to 42 kPa (2.8 to 6.0 PSI), refer to DTC P0106

• | With the engine idling, observe the MAF Sensor parameter. The MAF Sensor parameter should be between 2 to 6 g/s at idle.

⇒ | If the MAF Sensor parameter is not between 2-6 g/s at idle, refer to need link for (DTC P0101)

• | Vacuum hoses for splits, kinks, and improper connections

• | The air intake duct for being collapsed or restricted

• | The air filter for being dirty or restricted

• | Objects blocking the throttle body

• | Excessive fuel in the crankcase due to leaking fuel injectors

• | The EVAP control system for improper operation, refer to Evaporative Emission Control System Description

• | Excessive fuel pressure, refer to Fuel System Diagnosis

• | Malfunctioning fuel injectors, refer to Fuel Injector Diagnosis

• | Fuel contamination, refer to Alcohol/Contaminants-in-Fuel Diagnosis

• | The HO2S for improper installation and for electrical wires or connectors that may have contacted the exhaust system

• | The HO2S signal circuit shorted to voltage

Repair or replace as necessary.

### Repair Instructions

• | Air Cleaner Assembly Replacement

• | Air Cleaner Inlet Duct Replacement

• | need link for Evaporative Emission Canister Purge Solenoid Valve Replacement

• | Evaporative Emission Canister Replacement

• | need link for Fuel Injector Replacement

• | Heated Oxygen Sensor Replacement - Sensor 1

• | Heated Oxygen Sensor Replacement - Sensor 2

• | Mass Airflow Sensor Replacement

• | Throttle Body Assembly Replacement

### Repair Verification

Note: After repairs, use the scan tool fuel trim reset function in order to reset the long term fuel trim values.

1. Install any components or connectors that have been removed or replaced during diagnosis.
2. Perform any adjustment, programming, or setup procedures that are required when a component or module is removed or replaced.
3. Turn ON the ignition, with the engine OFF.

Note: Do not clear codes with the engine running. DTCs may reset in the same ignition cycle

4. Clear the DTCs.
5. Turn OFF the ignition for 60 s.
6. Start the engine.
7. Duplicate the conditions for running the DTC and use freeze frame / failure records, if applicable, in order to verify the DTC does not reset. If the DTC resets or another DTC is present, refer to Diagnostic Trouble Code (DTC) List - Vehicle
8. In order to ensure that the performance of the catalyst has not been affected by the fault that set this DTC, perform the repair verification for DTC P0420 00. Refer to DTC P0420