GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 GMC C Sierra - 2WD 🢒 Body and Accessories 🢒 Wiring Systems 🢒 DTC P0172 or P0175

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 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, this diagnostic trouble code (DTC) will set. If the PCM detects excessive vapor then a pass is logged.

Conditions for Running the DTC
    • DTCs P0101, P0103, P0108, P0135, P0137, P0141, P0200, P0300, P0410, P0420, P0430, P0440, P0442, P0443, P0446, P0449, P0506, P0507 or P1441 are not set.
    • The engine coolant temperature (ECT) is between 75-115°C (167-239°F).
    • The intake air temperature (IAT) is between -20 to +90°C (4-194°F).
    • The manifold absolute pressure (MAP) is between 26-90 kPa (3.7-13 psi).
    • The vehicle speed is less than 137 km/h (85 mph).
    • The engine speed is between 400-3,000 RPM.
    • The barometric pressure (BARO) is more than 74 kPa (10.7 psi).
    • The mass air flow (MAF) is between 5- 90 g/s.
    • The fuel level is more than 10 percent.
    • The throttle position (TP) is less than 90 percent

Conditions for Setting the DTC
    • The average long term fuel trim value is below 13 percent.
    • All of the above conditions are present for 40 seconds.

Diagnostic Aids
    • Fuel contamination, such as water and alcohol will effect fuel trim.
    • A malfunctioning mass air flow (MAF) 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.

  2. 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 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 ignition, with the engine OFF.
  6. Review the Freeze Frame Records/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?

-13%

Go to Step 3

Go to Diagnostic Aids

3
  1. Operate the engine at idle.
  2. Using the scan tool observe HO2S parameters.

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 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
    • Turn OFF the engine
    • Check the HO2S for proper installation.
    • Check to ensure electrical connectors and wires are secured and not contacting the exhaust system.

Did you find and correct the condition?

--

Go to Step 7

Go to Fuel System Diagnosis

6

Inspect for the following:

    • Check for excessive fuel in the crankcase.
    • Inspect the Evaporative Emissions Control System .
    • Test fuel pressure regulator for proper operation. Refer to Fuel System Diagnosis
    • Check 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 Symptoms - Engine Mechanical

7

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

  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 8

Go to Step 2

8

With a scan tool, observe the stored information, Capture Info.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic System Check - Engine Controls

System OK