P0172

Fuel Trim Cells:

Circuit Description

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 PCM monitors the HO2S 1 signal and adjusts fuel delivery based upon the HO2S signal voltages. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with a scan tool. Ideal fuel trim values are around 0%; if the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 0%. If a rich condition is detected, the fuel trim values will be below 0%, indicating that the PCM is reducing the amount of fuel delivered. If an excessively rich condition is detected, the PCM will set DTC P0172. The PCMs maximum authority to control long term fuel trim allows a range between -30% and +24%. The PCMs maximum authority to control short term fuel trim allows a range between -27% and +27%. The PCM monitors fuel trim under various operating conditions (fuel trim cells) before determining the status of the fuel trim diagnostic. The fuel trim cells are as illustrated.

The vehicle may have to be operated in all of the above conditions marked by an X, before the PCM determines fuel trim status. The amount of fuel trim deviates from 0% in each cell and the amount that each cell is used directly affects whether or not the vehicle must be operated in all of the cells described above to complete the diagnostic.

Conditions for Running the DTC

^ No MAF, ECT, TP, MAP, CMP, CKP, IAT sensor, EVAP, EGR, HO2S, Misfire, Injector circuit, Idle speed, VSS, Ignition Control (IC) circuit DTCs set.
^ Engine coolant temperature between 20°C (68°F) and 110°C (230°F).
^ Intake air temperature is between -18°C (0°F) and 70°C (158°F).
^ Manifold absolute pressure is between 15 kPa and 85 kPa.
^ Throttle angle is steady and less than 90%.
^ Vehicle speed is less than 132 km/h (82 mph).
^ Engine speed is between 600 and 4000 RPM.
^ Baro is greater than 70 kPa.
^ Air flow is between 3 gm/s and 150 gm/s.

Conditions for Setting the DTC

^ Long term fuel trim is at or near maximum authority of 24% (24% Cal).
^ Short term fuel trim is at or near maximum authority of 27% (27% Cal).
^ All conditions have been met in the Fuel Trim Cells.

Action Taken When the DTC Sets

^ The PCM will illuminate the MIL during the second consecutive trip in which the diagnostic has been run and failed.
^ If equipped with traction control, the PCM will command the Electronic Brake and Traction Control Module (EBTCM) via the serial data circuit to turn OFF traction control, and the EBTCM will illuminate the TRACTION OFF lamp.
^ The PCM will store conditions which were present when the DTC set as Freeze Frame and Fail Records data.

Conditions for Clearing the MIL/DTC

^ The PCM will turn the MIL OFF during the third consecutive trip in which the diagnostic has been run and passed.
^ The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
^ The DTC can be cleared by using a scan tool.

Diagnostic Aids

Check the following items:

^ Fuel pressure high - The system will go rich if pressure is too high. The PCM can compensate for some increase. However, if fuel pressure is too high, a DTC P0172 may be set. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
^ Fuel injector(s) faulty - Refer to Fuel Injector Balance Test. Component Tests and General Diagnostics
^ Check the EVAP Canister for Fuel Saturation. If the EVAP Canister is full of fuel, check canister control and hoses.
^ Disconnect the MAF sensor and see if rich condition is corrected. If so, replace the MAF sensor.
^ Check for a leaking fuel pressure regulator diaphragm by checking the vacuum line to the fuel pressure regulator for the presence of fuel.
^ An intermittent TP sensor output will cause the system to go rich due to a false indication of the engine accelerating.

Important: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

^ Poor terminal connection.
Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension. Refer to Diagrams.
^ Damaged harness.
Inspect the wiring harness for damage. If the harness appears to be OK, observe the sensor display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the sensor display may indicate the location of the fault.
^ Inspect the PCM and the engine grounds for clean and secure connections.

If the DTC is determined to be intermittent, reviewing the Fail Records can be useful in determining when the DTC was last set.

Test Description

The numbers below refer to the step numbers on the Diagnostic Table:

2. DTCs other than P0172 may indicate a condition present which may cause a lean condition. If this is the case, repairing the condition which caused the other DTC will most likely correct the DTC P0172.
4. If the DTC P0172 test passes while the Fail Records conditions are being duplicated, the rich condition is intermittent. Refer to Diagnostic Aids or Symptoms for additional information on diagnosing intermittent problems.