GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2001 🢒 Alero Export 🢒 DTC 🢒 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 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). 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 index is the average of short and long term fuel trim 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, this diagnostic trouble code (DTC) will set. If the PCM detects excessive vapor then a pass is logged.

Conditions for Running the DTC
    • DTCs P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0125 , P0131, P0132, P0133, P0134, P0201-P0204, P0300, P0301-P0304, P0325, P0341, P0446, P0502, P0503, P0506, P0507, P0601, P0602, P1133, or P1441 are not set.
    • The engine coolant temperature (ECT) is between 60-115°C (140-239°F).
    • The intake air temperature is between -25 to +115°C (-13 to +239°F).
    • The manifold absolute pressure (MAP) is above 26 kPa (4.6 psi).
    • The vehicle speed is less than 132 km/h (82 mph).
    • The engine speed is between 550-3,400 RPM.
    • The barometric pressure is more than 72 kPa (10.3 psi).
    • The fuel ethanol composition is less than 88 percent.
    • The fuel level is more than 9.8 percent.

Conditions for Setting the DTC
    • The Fuel Trim Index is below -42 percent.
    • 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 suppling to much fuel.
    • Use the scan tool in order to review the Failure Records. If intermittent condition is suspected , refer to Intermittent Conditions .

Test Description

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

  1. An 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.

DTC P0172-Fuel Trim System Rich

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 diagnosis.

  1. Install a scan tool.
  2. Start the engine and drive the vehicle under normal conditions until the engine reaches normal operating temperature and is in closed loop.
  3. Turn OFF the engine.
  4. Turn ON the ignition, with the engine OFF.
  5. Review the Freeze Frame Records and/or Failure Records and record displayed data for this DTC.

Does the scan tool indicate that the Fuel Trim Index is less than the specified value?

-42%

Go to Step 3

Go to Diagnostic Aids

3
  1. Operate the engine at idle.
  2. Using the scan tool, observe the O2S 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. 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 dirt or for restrictions
    • For objects blocking the throttle body

Did you find and correct the condition?

--

Go to Step 7

Go to Step 6

5

Turn OFF the engine.

  1. Check the O2S for proper installation.
  2. 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:

  1. Check for excessive fuel in the crankcase.
  2. Inspect the Evaporative Emissions Control System.
  3. Test fuel pressure regulator for proper operation. Refer to Fuel System Diagnosis .
  4. 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 or Fuel Injector Balance Test with Tech 2 .

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. 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 Trouble Code (DTC) List

System OK