P0172

Code P0172, Fuel Trim Rich (Part 1 Of 2):

Code P0172, Fuel Trim Rich (Part 2 Of 2):

Heated Oxygen Sensor (HO2S) Circuit:

CIRCUIT DESCRIPTION
To provide tile 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 oxygen sensor signal voltage and adjusts fuel delivery based on 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 the Tech 1. Ideal fuel trim values are around 128; if the oxygen sensor signal is indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 128. If a rich condition is detected, the fuel trim values will be below 128, indicating that the PCM is reducing the amount of fuel delivered. If an excessively rich condition is detected. the PCM will set DTC P0172.

DTC P0172 WILL SET WHEN:
^ DTC P0101, P0117, P0118, P0122, P0123, P0501 or P0502 not set.
^ Throttle angle less than 91%.
^ RPM between 1000 and 4000 RPM.
^ Engine coolant temperature between 80°C (176°F) and 110°C (230°F).
^ Purge duty cycle less than 80%.
^ Vehicle speed less than 70 mph.
^ Long term fuel trim is below 100.
^ Short term fuel trim is below 94.
^ Above conditions for over 25 seconds.

ACTION TAKEN (PCM will default to):
When DTC P0172 has initially been set, a software flag indicating an emissions-related fault will be set for 3 trips, after which the flag will clear provided the rich condition is no longer present. However, if conditions for setting DTC P0172 are detected during a trip while the flag is still set, the PCM will illuminate the MIL (Service Engine Soon). The MIL will remain illuminated for all subsequent trips until the DTC has been cleared by service or the rich condition has not been detected and conditions for checking fuel trim have been met for 3 consecutive trips.

NOTE: A trip is not considered complete until the fuel trim diagnostic has been run by the PCM. To complete a trip cycle for the fuel trim diagnostic, test must be enabled and run according to the "DTC P0172 Will Set When" criteria above.

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Visually/physically checking items which may cause a rich condition may determine the cause of DTC P0172 being set.
2. A mass air flow sensor that causes the PCM to read a higher than actual air flow can cause a rich condition and DTC P0172 to be set.
3. Silicon contamination of the Heated Oxygen Sensor (HO2S) can cause a false rich indication to be seen by the PCM. The PCM will then reduce the amount of fuel delivered, possibly causing severe driveability symptoms.


DIAGNOSTIC AIDS
If the problem cannot be isolated using the diagnostic charts, try monitoring L.T. fuel trim and fuel trim cell while operating the vehicle under various loads. This may isolate the condition which caused P0172 to be set.