GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2004 🢒 Grand Prix 🢒 Engine 🢒 Engine Controls - 3.8L (L26 and L32) 🢒 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 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 indicate 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 full 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. If the PCM detects an excessively lean condition, DTC P0171 sets.

Conditions for Running the DTC
    • DTCs P0030, P0036, P0068, P0101, P0102, P0103, P0107, P0108, P0120, P0130-P0138, P0140, P0141,P0201-P0206, P0220, P0300, P0401, P0403, P0404, P0405, P0420, P0442, P0446, P0449, P0452, P0453, P0455, P0462, P0463, P0506, P0507, P1404, P1516, P1518 are not set.
    • The engine is in Closed Loop status.
    • Fuel Trim Learn is enabled.
    • The engine coolant is more than -40°C (-40°F).
    • The intake air temperature (IAT) is more than - 18°C (-0.4°F).
    • The manifold absolute pressure (MAP) is between 25-199 kPa (3.6-28.8 psi).
    • The vehicle speed is less than 132 km/h (82 mph).
    • The engine speed is between 0-6,000 RPM.
    • The barometric pressure (BARO) is more than 74 kPa (10.7 psi).
    • The mass air flow (MAF) is between 3--150 g/s.
    • The fuel level is more than 10 percent.

Conditions for Setting the DTC
    • The average Long Term fuel trim is above 16.4 percent.
    • The above condition is present for more than 2 minutes.

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 providing enough fuel.
    • A lean condition could be present during high fuel demand due to a fuel pump not pumping enough fuel, plugged fuel filter or a restricted fuel line.
    • Insufficient fuel in the tank or low fuel pressure may cause this DTC to set. Refer to Fuel System Diagnosis .
    • 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-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Are any DTCs other than DTC P0171 also set?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3
  1. Install the scan tool.
  2. Turn ON the ignition.
  3. Observe the Freeze Frame/Failure Records parameter with a scan tool and record the displayed data for this DTC.
  4. Start and idle the engine at normal operating temperature in the Closed Loop.
  5. Observe the Long Term FT parameter.

Does the scan tool indicate that the Long Term fuel trim is more than the specified value?

16.4%

Go to Step 4

Go to Diagnostic Aids

4
  1. Operate the engine at idle.
  2. Observe the HO2S 1 and HO2S 2 parameters with a scan tool.

Does the scan tool indicate that the heated oxygen sensor (HO2S) voltages 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. • Vacuum hoses for splits, kinks, and proper connections--Refer to Emission Hose Routing Diagram .
    • Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

Did you find and correct the condition?

--

Go to Step 9

Go to Step 8

6
  1. Turn OFF the engine.
  2. Inspect the HO2S for proper installation. Refer to Heated Oxygen Sensor 1 Replacement and Heated Oxygen Sensor 2 Replacement .
  3. Ensure that the electrical connectors and wires are secured and not contacting the exhaust system.

Did you find and correct the condition?

--

Go to Step 9

Go to Step 7

7

Test for a short between the signal circuit and the low reference circuit or ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 9

Go to Fuel System Diagnosis

8
  1. Operate the engine at idle.
  2. Inspect for the following conditions:
    • Missing, loose, or leaking exhaust components, from the HO2S forward--Refer to Symptoms - Engine Exhaust in Engine Exhaust.
    • Vacuum leaks at the intake manifold, the throttle body, and the injector O-rings
    • Air induction system and the air intake ducts for leaks or for a missing air filter element--Refer to Air Cleaner Assembly Replacement and Air Cleaner Inlet Duct Replacement .
    • Evaporative canister purge lines plugged or obstructed
    • Properly functioning fuel injectors--Refer to Fuel Injector Solenoid Coil Test .
    • Crankcase ventilation system for leaks--Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical.

Did you find and correct the condition?

--

Go to Step 9

Go to Symptoms - Engine Mechanical in Engine Mechanical

9
  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 in 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 10

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

System OK