GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2006 🢒 GMC Sierra Denali 🢒 Restraints 🢒 SIR 🢒 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 PCM adds oxygen sensor inputs and level of purge to calculate short and long term fuel trim adjustments. If the oxygen sensors indicated 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. The values for the short term fuel trim 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. A block of cells contain information arranged in combinations of engine RPM and engine load for a fuel range of vehicle operating conditions. The long term fuel trim diagnostic is based on an average of cells currently being used. The PCM selects the cells based on the engine speed and engine load. 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 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, P0036, P0050, P0053, P0054, P0056, P0059, P0060, P0101, P0102, P0103, P0106, P0107, P0108, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0151, P0152, P0153, P0154, P0155, P0157, P0158, P0160, P0161, P0200, P0300, P0442, P0443, P0446, P0449, P0452, P0453, P0455, P0496, P0506, P0507, P1133, P1134, P1153, P1154, P2A01, P2A04 are not set.
    • The Fuel Trim Learn parameter is enabled.
    • The Loop Status parameter is closed.
    • The engine coolant temperature (ECT) is between -40 to +139°C (-40 to +282° F).
    • The intake air temperature (IAT) is between -20 to +152°C (-4 to +304°F).
    • The manifold absolute pressure (MAP) is between 15-105 kPa (2.2-15.2 psi).
    • The vehicle speed is less than 132 km/h (82 mph).
    • The engine speed is between 400-6,500  RPM.
    • The barometric pressure (BARO) is more than 70 kPa (10.2 psi).
    • The mass air flow (MAF) is between 1-250 g/s.
    • The fuel level is more than 15 percent.
    • This diagnostic runs continuously when the above conditions are met.

Conditions for Setting the DTC

The long term fuel trim value is more than a calibrated value for approximately 3 minutes after the Conditions for Running the DTC have been met.

Action Taken When the DTC Sets

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 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. The control module illuminates the malfunction indicator lamp (MIL) when one of the following occur:

    • The control module detects the same fuel trim failure during 2 consecutive trips.
    • The control module detects any fuel trim failure during any subsequent trip if the conditions at the time of failure meet the following criteria:
       - The engine load is within 20 percent of the previous test that failed.
       - The engine speed is within 375 RPM of the previous test that failed.
       - The engine coolant temperature is in the same range of the previous test that failed.

Conditions for Clearing the MIL/DTC
    • The control module turns OFF the malfunction indicator lamp (MIL) at the beginning of the fourth ignition cycle, after 3 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 and related Freeze Frame data clears after 40 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 MAF sensor can cause a rich condition and set this DTC. Refer to DTC P0101 .
    • Review Failure Records with a scan tool. If an intermittent condition is suspected , refer to Testing for Intermittent Conditions and Poor Connections .

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.

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Are any DTCs other than DTC P0172 or P0175 also set?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 3

3
  1. Install a 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/Failure Records and record the 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 4

Go to Diagnostic Aids

4
  1. Operate engine at idle.
  2. Observe Heated Oxygen Sensor (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 5

Go to Step 6

5
  1. Turn OFF the engine.
  2. Visually and physically inspect the following items:
    3. • The evaporative emission (EVAP) lines and components for damage or blockage--Refer to Evaporative Emissions Hose Routing Diagram .
    • The inlet screen of the mass air flow (MAF) sensor for blockage
    • The 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 the throttle body

Did you find and correct the condition?

--

Go to Step 8

Go to Step 7

6

  1. Turn OFF the 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 for continuity between the signal circuit and the low reference circuit. Refer to the following:

Did you find and correct the condition?

--

Go to Step 8

Go to Fuel System Diagnosis

7

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 8

Go to Symptoms - Engine Mechanical

8

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

  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 9

9

In order to ensure that the performance of the catalyst has not been affected by the fault that set this DTC, operate the vehicle within the conditions for running and setting DTC P0420 or P0430 and verify that P0420 or P0430 runs and passes. Refer to DTC P0420 or P0430 .

Does the DTC run and pass

--

Go to Step 10

Go to DTC P0420 or P0430

10

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

System OK