GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2004 🢒 Bonneville 🢒 Engine 🢒 Engine Controls - 3.8L (L36 and L67) 🢒 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 differently during Open and Closed Loop. During Open Loop the PCM determines fuel delivery based on sensor signals, without oxygen sensor input. During Closed Loop, the oxygen sensor inputs are added and used by the PCM to calculate Short and Long Term fuel trim fuel delivery adjustments. If the oxygen sensors indicate a lean condition, fuel trim values will be above 0 percent. If the oxygen sensors indicate a rich condition, fuel trim values will be below 0 percent. Short Term fuel trim values change rapidly in response to the heated oxygen sensor (HO2S) voltage signals. Long Term fuel trim makes coarse adjustments in order to maintain an air/fuel ratio of 14.7:1. The 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 will set. If the PCM detects excessive vapor, a pass is logged.

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0107, P0108, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0201, P0122, P0201-P0206, P0300, P0401, P0403, P0404, P0405, P0440, P0442, P0446, P0506, P0507, P1404 or P1441 are not set.
    • The engine coolant temperature (ECT) is between 10-126°C (50-259°F).
    • The intake air temperature (IAT) is between -18 and +140°C (-4 and +284°F).
    • The manifold absolute pressure (MAP) is between 15-105 kPa (2.1-15.2 psi).
    • The vehicle speed is less than 132 km/h (82 mph).
    • The engine speed is between 525-5,550 RPM.
    • The barometric pressure (BARO) is more than 74 kPa (10.7 psi).
    • The mass air flow (MAF) is between 2.7-150 g/s.
    • The fuel level is more than 10 percent.

Conditions for Setting the DTC
    • The average Long Term fuel trim value is below -18 percent.
    • All of the above conditions are present for 40 seconds.

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 effect fuel trim.
    • A malfunctioning mass air flow sensor can cause a rich condition and set this DTC. Refer to DTC P0101 .
    • Using scan tool, review Failure Records. If intermittent condition is suspected , refer to Intermittent Conditions .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. If conditions were not corrected, refer to Fuel System Diagnostic for possible fuel problem.

  1. An EVAP canister that is saturated will cause a rich condition. Fuel in the vacuum line to the fuel pressure regulator indicates a leaking 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 continuing.

  1. Install the scan tool.
  2. Start and idle the engine at the normal operating temperature in Closed Loop.
  3. Record the Long Term fuel trim data.
  4. Turn OFF the engine.
  5. Turn ON the ignition, with the engine OFF.
  6. Review the Freeze Frame/Failure Records, and record displayed data for this DTC.

Does the scan tool indicate that the Long Term fuel trim is less than the specified value?

-18%

Go to Step 3

Go to Diagnostic Aids

3
  1. Start and operate the engine at idle.
  2. Observe HO2S 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. • Inlet screen of the mass air flow (MAF) sensor for blockage
    • 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
    • Check 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. Visually and physically inspect the following items:
    3. • The heated oxygen sensor (HO2S) for proper installation
    • The electrical connectors and wires are secured and not contacting the exhaust system
  3. 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
    • Inspect the evaporative emissions (EVAP) control system.
    • Test fuel pressure regulator for proper operation. Refer to Fuel System Diagnosis .
    • Inspect to ensure that all injectors are functioning properly. Refer to Fuel Injector Solenoid Coil Test .
    • Perform the Fuel Injector Balance Test. Refer to Fuel Injector Balance Test with Special Tool .

Did you find and correct the condition?

--

Go to Step 7

Go to Fuel System Diagnosis

7

Important: After repairs, use 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 in 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