GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2003 🢒 GMC C Sierra - 2WD (CNG) 🢒 DTC 🢒 DTC P0171 or P0174

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. CNG 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 (O2S) input. During Closed Loop, the O2S inputs are added and used by the PCM to calculate short and long term fuel trim fuel delivery adjustments. If the O2S indicate a lean condition, fuel trim values will be above 0 percent. If the O2S 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 16.5:1 while operating on CNG. If the PCM detects an excessively lean condition, DTC P0171 or P0174 sets.

Conditions for Running the DTC
    • DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0125, P0200, P0300, P0327, P0332, P0335, P0336, P0351-P0358, P0401, P0402, P0403, P0443, P0446, P0449, P0496, P0502, P0503, P1020-P1028, or P1258 are not set.
    • The engine coolant temperature (ECT) is between 50-115°C (122-239°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 airflow (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 cell value is above 28 percent.
    • All of the above conditions exist for at least 2.5 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
    • The system will go lean if an injector is not suppling enough fuel.
    • A lean condition could be present during high fuel demand.
    • Use a scan tool in order to review Failure Records. If an 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 Diagnosis for a possible fuel problem.

    2.

  2. If conditions were not corrected, a worn cam, worn intake or exhaust valves, or other engine mechanical failure may be the problem.

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check Alternative Fuels - Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

2

Important: If any DTCs other than P0171 or P0174 are set, 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.
  4. Turn OFF the engine.
  5. Turn ON ignition, with 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 greater than the specified value?

28%

Go to Step 3

Go to Diagnostic Aids

3
  1. Operate engine at idle.
  2. Observe the HO2S parameters with a scan tool.

Does the scan tool indicate that the parameter is 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. • The vacuum hoses for splits, kinks, and proper connections. Refer to Emission Hose Routing Diagram in Engine Controls in the C/K Pickup Service Manual.
    • Ensure that the vehicle has sufficient fuel rail pressure. If the fuel rail pressure is too low this DTC may set. Refer to Fuel System Diagnosis .

Did you find and correct the condition?

--

Go to Step 7

Go to Step 6

5

  1. Turn OFF the engine.
  2. Inspect the HO2S for proper installation.
  3. Verify the electrical connectors and the wires are secure, and not contacting the exhaust system.
  4. Test for continuity between the HO2S signal circuit and the low reference circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 7

Go to Fuel System Diagnosis in Engine Controls in the C/K Pickup Service Manual

6

  1. Operate the engine at idle.
  2. Inspect for any missing, loose, or leaking exhaust components forward of the HO2S.
  3. Inspect for vacuum leaks at the intake manifold, throttle body, and injector O-rings.
  4. Inspect the air induction system and the air intake ducts for leaks.
  5. Inspect the crankcase ventilation system for leaks. Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical in the C/K Pickup Service Manual.

Did you find and correct the condition?

--

Go to Step 7

Go to Symptoms - Engine Mechanical in Engine Mechanical in the C/K Pickup Service Manual

7

Important: After repairs, use the 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 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