GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Envoy - 2WD 🢒 Engine 🢒 Engine Controls - 4.2L 🢒 DTC P0171

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 Loop 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 to calculate short and long term fuel trim (fuel delivery adjustments). Short Term fuel trim values change rapidly in response to the HO2S voltage signals. Long Term fuel trim makes coarse adjustments in order to maintain an air/fuel ratio of 14.7:1. Fuel Trim Index is the average of Short and Long Term Fuel Trim and Purge Learn Memory based on engine speed and load. If the PCM detects an excessively lean condition, this diagnostic trouble code (DTC) will set.

Conditions for Running the DTC
    • DTCs P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0125 , P0131, P0132, P0133, P0134, P0201, P0202, P0203, P0204, P0300, 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 550-3400 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 above 23 percent.

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 due to a fuel pump that does not pump enough fuel.
    • Using a scan tool, review the 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 failures 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 P0171, refer to those DTCs before continuing.

  1. Connect the scan tool.

    2. Caution: Road test a vehicle under safe conditions and while obeying all traffic laws. Do not attempt any maneuvers that could jeopardize vehicle control. Failure to adhere to these precautions could result in serious personal injury and vehicle damage.

  2. Start the engine and drive the vehicle under normal conditions until the engine reaches 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/Failure Records and record displayed data for this DTC.

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

23%

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 .
    • If fuel pressure is too low, this DTC may set. Refer to Fuel System Diagnosis .
    • Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

Did you find and correct the condition?

--

Go to Step 7

Go to Step 6

5

  1. Turn OFF the engine.
    2. • Check the O2S for proper installation.
    • Check to ensure electrical connectors and wires are secured and not contacting the exhaust system.
  2. Check for short between the signal circuit and low reference circuit.

Did you find and correct the condition?

--

Go to Step 7

Go to Fuel System Diagnosis

6

Operate the engine at idle.

    • Check for missing, loose, or leaking exhaust components.
    • Check for vacuum leaks at the intake manifold, throttle body, and injector O-rings.
    • Check the air induction system and air intake ducts for leaks.
    • Check crankcase ventilation system for leaks. Refer to Crankcase Ventilation System Inspection/Description in Engine Mechanical.

Did you find and correct the condition?

--

Go to Step 7

Go to Symptoms - Engine Mechanical in Engine Mechanical

7

Important: After any of the above repairs, use 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