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 HO2S 1 signal and adjusts fuel delivery based upon the HO2S signal voltage. A change made to fuel delivery will be indicated by the long and Short Term FT 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 PCM's maximum authority to control Long Term FT allows a range between -23% and +16%. The PCM's maximum authority to control short term fuel trim allows a range between -11% 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 used to determine fuel trim are as follows:

- Idle, purge commanded.
- Deceleration, purge commanded.
- Normal driving, purge commanded.
- Deceleration, no purge commanded.
- Normal driving, no purge commanded.

The vehicle may have to be operated in all of the above conditions before the PCM determines fuel trim status. The amount that fuel trim deviates from 0% in each cell and the amount that each cell is weighted 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 MAF sensor, ECT sensor, HO2S, TP sensor, EVAP, EGR, Misfire, Injector circuit, Idle speed, CKP sensor, CMP Sensor, Vehicle speed, EST, MAP sensor, or IAT sensor 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).
- Barometric Pressure (BARO) is greater than 70 kPa.
- Manifold absolute pressure is between 18 kPa and 80 kPa.
- Throttle angle is less than 90%.
- Vehicle speed is less than 82 mph.
- Mass air flow is between 3 gm/s and 150 gm/s.
- Engine speed between 600 and 4000 rpm.
- Long Term FT is at or near maximum authority of +16%.
- Short Term FT is at or near maximum authority of +20%.
- All conditions have been met in 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 the Scan Tool.

Diagnostic Aids

Check the following items:

- Fuel Pressure. 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 Pressure Test. Fuel System Pressure Test
- Fuel injector(s). Refer to Fuel Injector Balance Test. Component Tests and General Diagnostics
- Check the EVAP Canister for Fuel Saturation. If 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 vacuum line to regulator for the presence of fuel. If a problem is found, replace the fuel pressure regulator.
- An intermittent TP sensor output will cause the system to go rich due to a false indication of the engine accelerating.
- Poor connection at 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 fault.

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed 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 number(s) below refer to the step number(s) 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 for additional information on diagnosing intermittent conditions.