P0172
Diagnostic Chart:
Circuit Description
In order 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 heated oxygen sensor signal voltage and adjusts fuel delivery based on signal voltage. A change made to fuel delivery will be indicated by long and short term fuel trim values which can be monitored with the scan tool. Ideal fuel trim values are around 0%. If the heated oxygen sensor 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. If exhaust emissions reach an excessive level due to a lean or rich condition, a fuel trim DTC is set.
Conditions for Setting the DTC
- Long term and short term fuel trim less than or equal to -22% in 128 firing events.
- Short term fuel trim less than or equal to -11% in 128 firing events.
- Barometric pressure less than 75 kPa.
Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the fault active.
- The PCM will record operating conditions at the time the diagnostic fails. This information will be stored in the Freeze Frame buffer.
Conditions for Clearing the MIL/DTC
- The MIL will turn off after three consecutive drive cycles without a fault present.
- A DTC will clear after 40 consecutive warm-up cycles without a fault.
- The DTCs can be cleared by using a scan tool or by disconnecting the PCM battery feed.
Diagnostic Aids
Check for any of the following conditions:
- Fuel pressure. The fuel system will be rich if the fuel pressure is too high. It may be necessary to monitor the fuel pressure while driving the vehicle at various road speeds. Refer to Fuel System Diagnosis. Table A-7B Fuel System Diagnosis (Fuel Pressure Check)
- MAF sensor poor performance.
- A poor connection at the PCM. Inspect the wiring harness connectors for backed out terminal, improper mating, broken locks, improperly formed or damaged terminals, and a poor terminal to wire connection.
- A damaged wiring harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the HO2S display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the HO2S display will indicate the location of the fault.
After all the checks have been completed and a malfunction continues to occur, replace the PCM with a known good unit and retest.
If the DTC P0172 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions since the DTC was last set. This may assist in determining how often the DTC sets.
If any wiring repairs need to be made, Refer to Repair Procedures in Diagrams/Electrical.
Test Description
The numbers below refer to the step numbers in the Diagnostic Table.
1. The Powertrain (OBD) System Check prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
2. If DTCs other than Fuel Trim P0171, P0172, Misfire P0300 to P0304 and EGR P0400 are set, diagnose those DTCs first.
3. Review the Freeze Frame data to determine when the DTC set. Always record this information.
8. If the condition is still present, any one of these sensors could cause this DTC to set. Check the performance of the MAF sensor, the ECT sensor, the TP sensor and the CTP switch signal. Repair any faulty conditions.