GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2005 🢒 XLR 🢒 Accessories 🢒 Navigation Systems 🢒 DTC P0172 or P0175

Circuit Description

The engine control module (ECM) 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 ECM determines fuel delivery based on sensor signals without oxygen sensor (O2S) input. During Closed Loop, the oxygen sensor inputs are added and used by the ECM to calculate short and long term fuel trim fuel delivery adjustments. If the O2S indicate a lean condition, the fuel trim values will be above 0 percent. If the O2S indicate a rich condition, the 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 ECM detects an excessively rich condition, DTC P0172 or P0175 sets.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC P0172 Fuel Trim System Rich Bank 1
    • DTC P0175 Fuel Trim System Rich Bank 2

Conditions for Running the DTC
    • DTCs P0030-P0032, P0036, P0038, P0050-P0052, P0056-P0058, P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0128, P0131-P0135, P0137-P0141, P0151-P0155, P0157-P0161, P0221, P0222, P0223, P0300, P0301-P0308, P0335, P0340, P0341, P0345, P0346, P0365, P0366, P0390, P0391, P0443, P0446, P0458, P0459, P1111, P1112, P1114, P1115, P1137, P1138, P1157, P1158, P1380, P1381, P2135, P2231, P2234 are not set.
    • Closed Loop is active.
    • The engine coolant temperature (ECT) is more than 70°C (158°F).
    • The intake air temperature (IAT) is more than -7°C (+19.4°F).
    • The vehicle speed is less than 137 km/h (85 mph).
    • The engine speed is more than 500 RPM.
    • The barometric pressure (BARO) is more than 70 kPa (10.1 psi).
    • The fuel level is more than 9.8 percent.
    • DTC P0172 and P0175 run continuously once the above conditions have been met.

Conditions for Setting the DTC
    • The combined value of the Short Term FT and the Long Term FT parameters are less than -25 percent.
    • The above condition is present for more than 100 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 or alcohol will effect fuel trim.
    • A malfunctioning mass air flow sensor can cause a rich condition and set this DTC. Refer to DTC P0101 .
    • An EVAP canister that is saturated will cause a rich condition. If the conditions were not corrected, a worn cam shaft, worn intake valves, worn exhaust valves, or other engine mechanical failure may be the problem.
    • Review the Failure Records with a scan tool. If an intermittent condition is suspected , refer to Intermittent Conditions .

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle?

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Go to Step 2

Go to Diagnostic Starting Point - Vehicle in Vehicle DTC Information

2

Important: If any DTCs other than P0172 are set, refer to those DTCs before continuing.

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

Does the scan tool indicate that the combined value of the short term and the long term fuel trim is less than the specified value?

-25%

Go to Step 3

Go to Diagnostic Aids

3
  1. Operate engine at idle.
  2. Observe heated oxygen sensor (HO2S) bank 1 sensor 1 and bank 2 sensor 1 parameters with a scan tool.

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

0.96-1.04

Go to Step 4

Go to Step 5

4
  1. Turn OFF engine.
  2. Visually and physically inspect the following items:
    3. • The evaporative emission (EVAP) lines and components for damage or blockage
    • The inlet screen of the mass air flow (MAF) sensor for blockage
    • The vacuum hoses for splits, kinks, and proper connections
    • The air intake duct for being collapsed or restricted
    • The air filter for being dirty or restricted
    • Inspect for objects blocking the throttle body.

Did you find and correct the condition?

--

Go to Step 7

Go to Step 6

5

  1. Turn OFF engine.
  2. Inspect the HO2S for proper installation.
  3. Inspect to ensure that the electrical connectors and the wires are secure and not contacting the exhaust system.
  4. Test the HO2S reference voltage current circuit for a short to the following circuits:
    5. • The input pump current circuit
    • The output pump current circuit

Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 7

Go to Step 6

6

Inspect for the following:

    • Excessive fuel in the crankcase
    • Proper operation of the fuel pressure regulator--Refer to Fuel System Diagnosis .
    • 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 Fuel System Diagnosis

7

Important: After repairs, reset the long term fuel with the scan tool Fuel Trim Reset function.

  1. Turn ON the ignition, with the engine OFF.
  2. Clear the DTCs with a scan tool.
  3. Turn OFF the ignition for 30 seconds.
  4. Start the engine.
  5. 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 - Vehicle in Vehicle DTC Information

System OK