GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2005 🢒 STS 🢒 Engine 🢒 Engine Controls - 4.6L (LH2) 🢒 DTC P0171 or P0174

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 O2S inputs are added and used by the ECM to calculate short and long term fuel trim (FT) fuel delivery adjustments. If the O2S indicates a lean condition, FT values will be above 0 percent. If the O2S indicate a rich condition, FT values will be below 0 percent. Short term FT values change rapidly in response to the heated oxygen sensor (HO2S) voltage signals. Long term FT makes coarse adjustments in order to maintain an air/fuel ratio of 14.7:1. If the ECM detects an excessively lean condition, DTC P0171 or P0174 sets.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC P0171 Fuel Trim System Lean Bank 1
    • DTC P0174 Fuel Trim System Lean 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, 2234 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 740 kPa (107.3 psi).
    • The mass air flow (MAF) is more than 60 mgstk.
    • The fuel level is more than 9.8 percent.
    • DTC P0171 and P0174 run continuously when 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 more 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
    • Excessive resistance in the fuel injector ignition 1 voltage circuit or the fuel injector control circuit may cause the following symptoms:
       - Lean condition
       - Misfire
       - Rough idle
    • The system will go lean if an injector is not suppling enough fuel.
    • A lean condition could be present during high fuel demand.
    • A malfunctioning MAF sensor can cause this DTC to set. Refer to DTC P0101 .
    • Review the Failure Records with a scan. 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 System Check - Vehicle in Vehicle DTC Information

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 short term and long term fuel trim (FT).
  4. Turn OFF the engine.
  5. Turn ON the 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 greater than the specified value?

25%

Go to Step 3

Go to Diagnostic Aids

3
  1. Operate the engine at idle.
  2. Observe the 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 the engine.
  2. Visually and physically inspect the following items:
    3. • The vacuum hoses for splits, kinks, and proper connections--Refer to Evaporative Emissions Hose Routing Diagram .
    • Ensure that the vehicle has sufficient fuel in tank. 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. Inspect the HO2S for proper installation.
  3. Verify the electrical connectors and the wires are secure, and not contacting the exhaust system.
  4. Test the HO2S reference voltage 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 Fuel System Diagnosis

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.

Did you find and correct the condition?

--

Go to Step 7

Go to Symptoms - Engine Mechanical in Engine Mechanical

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?

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Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

System OK