P0172

CIRCUIT DESCRIPTION
While in Closed Loop, the Engine Control Module (ECM) monitors the Heated Oxygen Sensor (HO2S) signal voltage and adjusts the fuel delivery based on the signal voltage. A change made to fuel delivery will be indicated by the Long and Short Term Fuel Trim values. The Short Term Fuel Trim values change rapidly in response to the HO2S signal voltages. These changes tine tune the engine fueling. The Long Term Fuel Trim values change in response to trends in the Short Term Fuel Trim. The Long Term Fuel Trim makes coarse adjustments to the fueling in order to re-center and restore control to the Short Term Fuel Trim. The Short and the Long Term Fuel Trim can be monitored with a scan tool. Ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the ECM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition. If an excessively rich or lean condition is detected, the ECM will set a Diagnostic Trouble Code (DTC). The Long Term Fuel Trim diagnostic parameter is an average of several of the long term speed load learn cells which the ECM selects based on the engine speed and the engine load.

CONDITIONS FOR RUNNING THE DTC
^ Closed Loop operation
^ Fuel trim enabled

CONDITIONS FOR SETTING THE DTC
The ECM detects the average Long Term Fuel Trim cell values are below a predetermined threshold.

ACTION TAKEN WHEN THE DTC SETS
^ The control module illuminates the Malfunction Indicator Lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
^ The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module turns OFF the Malfunction Indicator Lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
^ A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
^ A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
^ Use a scan tool in order to clear the MIL and the DTC.

DIAGNOSTIC AIDS
In terms of fuel trim, a value less than -8.5 percent would be -9.0 percent. This value indicates that the ECM is leaning the fuel mixture, or reducing the fuel in order to compensate for a rich condition. A positive value indicates the ECM is richening the fuel mixture, or adding more fuel in order to compensate for a lean condition. If using the diagnostic table does not isolate the problem, try monitoring the Long Term Fuel Trim cell while operating the vehicle under various loads. Monitor the engine data list for the possibility of a skewed sensor causing a driveability concern. This may isolate the condition which caused the setting of the current DTC. IP DTCs P0172 and P0175 are both set, inspect the fuel pressure regulators atmospheric vent hose for signs of fuel. The vent hose connects to the bottom side of the throttle body. If the fuel pressure regulator has failed the engine will burn the excess fuel causing a rich condition. A fault in the EVAP system could also cause a rich condition. Inspect the Evaporative Emission (EVAP) system for proper operation.

Use the J 35616-A Connector Test Adapter kit for any test that requires probing the ECM harness connector or a component harness connector. Using this kit prevents damage to the harness or component terminals. Refer to Using Connector Test Adapters in Diagrams. For an intermittent, refer to Symptoms - Computers and Control Systems. If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Diagrams. Symptom Related Diagnostic Procedures

TEST DESCRIPTION

Steps 1-9:

The numbers below refer 10 the step numbers on the diagnostic table.
2. This step determines if the fault is present.
3. This process may help in order to recreate the malfunction.
4. This step determines if both banks are running rich.
5. This step addresses areas of concern that could cause both banks of the engine to run rich.