P0172

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 1 HO2S 1 signals 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 on bank 1, the PCM will set DTC P0172. The PCMs maximum authority to control long term fuel trim allows a range between -20% and +20%. The PCMs maximum authority to control short term fuel trim allows a range between -20% and +20%. 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 shown in the illustration.

The vehicle may have to be operated in all of the above conditions 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 Setting the DTC
^ No VSS, EST, CKP, CAM, TP, Misfire, Fuel Injector, MAF, 02, MAP, EGR, EVAP, ECT or IAT DTCs are 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.
^ Long term fuel trim is at or near maximum authority of -20%
^ Short term fuel trim is at or near maximum authority of -20%.
^ 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 test has been run and failed.
^ 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:
The system will go rich if pressure is too low. The PCM can compensate for some decrease. However, if fuel pressure is too low, a DTC P0172 may be set. Refer to Fuel System Pressure Test (VIN 1). Fuel System Pressure Test
^ Fuel injector(s):
Refer to Fuel in] Coil Test - ECT Between 10-35 Degrees C or Fuel lnj Coil Test - ECT Outside 10-35 Degrees C. Component Tests and General Diagnostics
^ EVAP Canister:
Check the EVAP canister for fuel saturation. If the canister is full of fuel, visually and physically check the EVAP and fuel system components for damage. Refer to EVAP Control System Diagnosis. Component Tests and General Diagnostics
^ Mass Air Flow Sensor:
Disconnect the MAF sensor and see if rich condition is corrected. If so, replace the MAF sensor.
^ Fuel Pressure Regulator:
Check for a leaking fuel pressure regulator diaphragm by checking vacuum line to regulator for the presence of fuel. If a problem is found, replace the fuel pressure regulator.
^ TP Sensor:
An intermittent TP sensor output will cause the system to go rich due to a false indication of the engine accelerating. Throttle Angle displayed on a scan tool should increase steadily from 0% to 100% when opening the throttle.
^ Poor connection at the PCM:
Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
^ Damaged harness:
Inspect the wiring harness for damage. If the harness appears to be OK, observe the HO2S 1 display on the scan tool while moving connectors and wiring harnesses related to the engine harness. A change in the display will indicate the location of the malfunction.

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

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 rich 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 for additional information on diagnosing intermittent conditions.