P0171

DTC P0171 or P0172

Diagnostic Instructions

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

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

* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC P0171
- Fuel Trim System Lean

DTC P0172
- 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. The ECM monitors the heated oxygen sensor (HO2S) signal voltage and adjusts the fuel delivery based on the signal voltage while in Closed Loop. A change made to the fuel delivery changes the long and short term fuel trim (FT) values. The short term FT values change rapidly in response to the HO2S voltage signals. These changes fine tune the engine fueling. The long term FT makes coarse adjustments in order to maintain an optimum air/fuel ratio. The ideal FT values are around zero percent. A positive FT value indicates that the ECM is adding fuel in order to compensate for a lean condition. A negative FT value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition.

Conditions for Running the DTC

* DTCs P0030, P0053, P0101, P0102, P0103, P0106, P0107, P0108, P0131, P0132, P0133, P0134, P0135, P015A, P015B, P0178, P0179, P0201-P0204, P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0300, P0301-P0304, P0411, P0442, P0455, P0496, P0506, P0507, P1248, P1249, P124A, P124B, P2147, P2148, P2150, P2151, P2153, P2154, P2156, P2157, P2227, P2228, P2229, P2230, P2269, P2440, or P2444 are not set.

* The engine is in Closed Loop status.

* The catalyst monitor intrusive test, post O2 sensor intrusive test, EVAP leak test, and device controls are not active.

* The engine coolant temperature (ECT) is between -20 and +130°C (-4 and +266°F).

* The intake air temperature (IAT) is between -20 and +150°C (-4 and +302°F).

* The manifold absolute pressure (MAP) is between 15-255 kPa (2.2-37 PSI).

* The engine speed is between 400-6,600 RPM.

* The mass air flow (MAF) is between 1-512 g/s.

* The barometric pressure (BARO) is greater than 70 kPa (10.2 PSI).

* The fuel level is greater than 10 percent.

* These DTCs run continuously when the above conditions have been met.

Conditions for Setting the DTC

* The long term FT weighted average value is more or less than a calibrated value.

* The above condition is present for approximately 3 minutes after the conditions for running the DTC have been met.

Action Taken When the DTC Sets

DTCs P0171 and P0172 are Type B DTCs.

Conditions for Clearing the MIL/DTC

DTCs P0171 and P0172 are Type B DTCs.

Diagnostic Aids

Note: EVAP purge enablement may cause the FT to be momentarily outside the normal range.

* The normal Short Term FT parameter should be between +10 percent and -10 percent, with near 0 percent the optimum, with the engine running at operating temperature.

* The Long Term FT parameter should be between +10 percent and -10 percent, with near 0 percent the optimum, with the engine running at operating temperature.

* Inspect the air filter for being the correct one for this application. Make sure that the engine oil fill cap is in place and that it is tight. Verify that the engine oil dip stick is fully seated.

* Certain aftermarket air filters may cause a DTC to set.

* Certain aftermarket air induction systems or modifications to the air induction system may cause a DTC to set.

* Certain aftermarket exhaust system components may cause a DTC to set.

Reference Information

Electrical Information Reference

* Circuit Testing Circuit Testing

* Connector Repairs Connector Repairs

* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections

* Wiring Repairs Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References Control Module References for scan tool information

Circuit/System Verification

1. Verify that other DTCs are not set.

• If any other DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle Diagnostic Trouble Code (DTC) List - Vehicle.

2. Ignition ON, engine OFF, observe the manifold absolute pressure (MAP) sensor parameter. The MAP sensor pressure should be within the range specified for your altitude. Refer to Altitude Versus Barometric Pressure Specifications.

• If the MAP sensor does not indicate the correct barometric pressure, refer to DTC P0106 P0106.

3. With the engine idling, observe the mass air flow (MAF) sensor parameter. The MAF sensor parameter should be between 2-6 g/s at idle.

• If the MAF sensor parameter is not between 2-6 g/s at idle, refer to DTC P0101 P0101 or DTC P0102 or P0103 P0102.

4. Allow the engine to reach operating temperature. With the engine running, observe the affected Long Term FT parameter with a scan tool. The reading should be between -22 % and 38.5 %.

• If not within the specified range, verify none of the conditions listed below exist:

P0171

* Vacuum hoses split, kinked, or improperly connected

* Insufficient fuel in the tank

* Low fuel pressure. Refer to Fuel System Diagnosis Fuel System Diagnosis.

* Fuel contamination. Refer to Alcohol/Contaminants-in-Fuel Diagnosis Testing and Inspection.

* Malfunctioning fuel injectors. Refer to Fuel Injector Solenoid Coil Test Fuel Injector Solenoid Coil Test.

* Missing, loose, or leaking exhaust components from the HO2S forward.

* Vacuum leaks at the intake manifold, the throttle body, and the injector O-rings

* Leaks in the air induction system or the air intake ducts

* Missing air filter element

* Cracked EVAP canister

* Evaporative pipes obstructed or leaking

* Leaks in the crankcase ventilation system. Refer to Crankcase Ventilation System Inspection/Diagnosis (With Turbocharger) Testing and Inspection.

P0172

* Vacuum hoses split, kinked, or improperly connected

* The air intake duct collapsed or restricted

* Dirty or restricted air filter

* Objects blocking the throttle body

* Excessive fuel in the crankcase due to leaking fuel injectors

* Improperly operating EVAP control system

* Excessive fuel pressure. Refer to Fuel System Diagnosis Fuel System Diagnosis.

* Malfunctioning fuel injectors. Refer to Fuel Injector Solenoid Coil Test Fuel Injector Solenoid Coil Test.

* Fuel contamination. Refer to Alcohol/Contaminants-in-Fuel Diagnosis Testing and Inspection.

5. If all conditions test normal, test the engine for a mechanical condition. Refer to Symptoms - Engine Mechanical Symptoms - Engine Mechanical.

6. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

Repair Instructions

Perform the Diagnostic Repair Verification Verification Tests after performing the diagnostic procedure.