P0172

Diagnostic Chart:

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 fuel trim values which can be monitored with a scan tool. Ideal fuel trim values are around 0% if the HO2S signal is 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. It an excessively rich condition is detected on, the PCM will set DTC P0172.

The PCMs maximum authority to control long term fuel trim allows a range between -23% and +16%. The PCMs 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 are weighted (assigned a percentage of weight in determining total fuel trim status for the diagnostic) as follows:

^ Idle, purge commanded (Cell 0) - approximately 10%

^ Deceleration, purge commanded (Cell 1) - approximately 14%

^ Normal driving, purge commanded (Cell 2) approximately 26%

^ Deceleration, no purge commanded (Cell 6) - approximately 15%

^ Normal driving, no purge commanded (Cell 7) - approximately 35%

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, Throttle Position (TP) sensor, EVAP, EGR, Misfire, Injector circuit, Idle speed, MAP sensor, Vehicle Speed Sensor (VSS), Camshaft Position (CMP) sensor, Crankshaft Position (CKP) sensor, IC circuit or IAT sensor DTC(s) set.

^ Engine coolant temperature between 20°C (68°F) and 110°C (230°F).

^ Intake air temperature is between -18°C (0°F) and 65°C (149°F).

^ Manifold absolute pressure is between 18 kPa and 80 kPa.

^ Throttle angle is steady below 90%.

^ Vehicle speed is below 75 m.p.h.

^ Engine speed between 550 and 5000 RPM.

^ Long term fuel trim is at or near maximum authority of -23%.

^ Short term fuel trim is at or near maximum authority of -11%.

Action Taken When the DTC Sets

^ The PCM will illuminate the Malfunction Indicator Lamp (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 Clear Info function or by disconnecting the PCM battery feed.

Diagnostic Aids

Check for the following conditions:

^ 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 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.