GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2004 🢒 Rainier 🢒 Engine 🢒 Engine Controls - 4.2L 🢒 DTC P0172

Circuit Description

The powertrain control module (PCM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy and emission control. Fuel delivery is controlled during Open and Closed loop. During Open Loop the PCM determines fuel delivery based on sensor signals, without oxygen sensor (O2S) input. During Closed Loop, the O2S inputs are added and used by the PCM to calculate Short and Long Term Fuel Trim (FT), fuel delivery adjustments. Short Term FT values change rapidly in response to the heated oxygen sensor (HO2S) voltage signals. Long Term FT values change slower in response to trends in Short Term FT adjustments. The Fuel Trim Index is the average of Short and Long Term FT and Purge Learn Memory based on engine speed and load. Fuel trim diagnostic will conduct a test to determine if a rich failure actually exists or if excessive vapor from the evaporative emission (EVAP) canister is causing a rich condition. If the PCM detects an excessively rich condition, DTC P0172 sets.

Conditions for Running the DTC
    • DTCs P0013, P0014, P0016, P0068, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0120, P0122, P0123, P0125 , P0128, P0130, P0131, P0132, P0133, P0134, P0135, P0201-P0206, P0220, P0300, P0301-P0304, P0326, P0327, P0341, P0446, P0483, P0486, P0502, P0503, P0506, P0507, P0601, P0602, P0604, P0606, P0641, P0651, P1133, P1516, P1680, P1681, P2101, P2120, P2125, P2135, P2138, P2176 are not set.
    • The engine coolant temperature (ECT) is 60-125°C (140-257°F).
    • The intake air temperature (IAT) is -25 to +150°C (-13 to +302°F).
    • The manifold absolute pressure (MAP) is above 6 kPa (0.9 psi).
    • The vehicle speed is less than 132 km/h (82 mph).
    • The engine speed is between 400-5,850 RPM.
    • The barometric pressure (BARO) is more than 72 kPa (10.3 psi).
    • The fuel level is more than 10 percent.

Conditions for Setting the DTC
    • The Fuel Trim Index is below -35 percent.
    • The diagnostics have determined that a rich condition was not due to excessive vapor.

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.
    • Clear the MIL and the DTC with a scan tool.

Diagnostic Aids
    • Fuel contamination, such as water and alcohol, will affect the fuel trim.
    • The system will go rich if the injectors are supplying too much fuel.
    • Using the scan tool, review the Failure Records. If an intermittent condition is suspected , refer to Intermittent Conditions .

Test Description

The number below refers to the step number on the diagnostic table.

  1. An evaporative emission (EVAP) canister that is saturated will cause a rich condition. Fuel in the vacuum line to the fuel pressure regulator indicates a bad regulator. If conditions were not corrected, a worn cam, worn intake or exhaust valves or other engine mechanical failure may be at fault.

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check - Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Important: : If any DTCs are set other than P0172, refer to those DTCs before proceeding with this diagnostic.

  1. Install a scan tool.
  2. Review the Freeze Frame/Failure Records and record displayed data for this DTC.
  3. Select Fuel Trim Data parameter.
  4. Start the engine and observe the Fuel Trim Index.

Is the Fuel Trim Index parameter less than the specified value?

-35%

Go to Step 3

Go to Diagnostic Aids

3
  1. Start and operate the engine at idle.
  2. Observe O2S parameters with a scan tool.

Does the scan tool indicate that the values are within the specified range and fluctuating?

200-800 mV

Go to Step 4

Go to Step 5

4
  1. Turn OFF the engine.
  2. Visually and physically inspect the following items:
    3. • Vacuum hoses for splits, kinks, and proper connections--Refer to Emission Hose Routing Diagram .
    • The air intake duct for being collapsed or restricted
    • The air filter for being dirty or restricted
    • For objects blocking throttle body

Did you find and correct the condition?

--

Go to Step 7

Go to Step 6

5
  1. Turn OFF the engine.
  2. Inspect the oxygen sensor (O2S) for proper installation.
  3. Ensure electrical connectors and wires are secured and not contacting the exhaust system.
  4. Test for continuity between the signal circuit and the low reference circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 7

Go to Fuel System Diagnosis

6

Inspect for the following:

    • Excessive fuel in the crankcase
    • The evaporative emissions (EVAP) control system
    • The fuel pressure regulator for proper operation--Refer to Fuel System Diagnosis .
    • Ensure that all injectors are functioning properly. Refer to Fuel Injector Solenoid Coil Test .

Did you find and correct the condition?

--

Go to Step 7

Go to Symptoms - Engine Mechanical in Engine Mechanical

7

Important: After repairs, use the scan tool Fuel Trim Reset function in order to reset the Long Term Fuel Trim.

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 8

8

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK