GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2005 🢒 LaCrosse 🢒 Restraints 🢒 SIR 🢒 DTC P2178

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

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P2178 Fuel Trim System Rich at Cruise or Accel

Conditions for Running the DTC
    • Before the ECM can report DTC P2178 failed, DTCs P0030, P0031, P0032, P0053, P0101, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0135, P0221, P0222, P0223, P0335, P0336, P0338, P0443, P0458, P0459, P167A, P2096, P2097, P2196, P2231, P2237, P2243, P2251, and P2626 must run and pass.
    • DTCs P0010, P0011, P0013, P0014, P0020, P0021, P0023, P0024, P0455, P0496, P0117, P0118, P0125, P2088, P2089, P2090, P2091, P2092, P2093, P2094, P2095, and P2176 are not set.
    • The fuel system is in closed loop.
    • The long fuel trim is active.
    • The engine coolant temperature (ECT) is more than 60°C (140°F).
    • The evaporative emission (EVAP) canister purge solenoid valve is not enabled.
    • The intake air temperature (IAT) is less than 60°C (140°F).
    • The fuel level is more than 10.5 percent.
    • The amount of accumulated air flow into the engine is more than 7000 grams.
    • DTC P2178 runs continuously once the above conditions are met for at least 300 seconds.

Conditions for Setting the DTC
    • The Total Fuel Trim Avg. is less than -22 percent.
    • The condition exists for 4 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 4 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
    • A fuel delivery condition causes this DTC to set. Thoroughly inspect all items that cause a rich condition.
    • For an intermittent condition, refer to Intermittent Conditions .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This step determines whether the fault is present.

  2. Disconnecting the MAF sensor determines if the MAF sensor signal is skewed. If the Short Term FT parameter changes more than the specified value, there is a condition with the MAF sensor. A MAF sensor condition can cause this DTC without setting a MAF DTC. If there is a MAF sensor condition, the MAF sensor parameters will appear to be within range.

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

Important: If any DTCs are set, except P2178, refer to those DTCs before proceeding with this diagnostic.

  1. Idle the engine at the normal operating temperature.
  2. The fuel system is in Closed Loop.
  3. Observe the Total Fuel Trim Avg. parameter with a scan tool.

Is the Total Fuel Trim Avg. less than the specified value?

-22%

Go to Step 3

Go to Step 4

3
  1. Observe the Freeze Frame and/or the Failure records data for this DTC.
  2. Turn the ignition OFF 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 and/or the Failure records data.

Does the DTC fail this ignition cycle?

--

Go to Step 4

Go to Diagnostic Aids

4

  1. Start the engine.

    2. Important: Additional DTCs set with the mass air flow (MAF) sensor disconnected.

  2. Disconnect the MAF sensor harness connector while the engine is operating.
  3. Observe the Short Term FT parameter with a scan tool.
  4. Connect the MAF sensor after completing this step.

Does the Short Term FT parameter change more than the specified value with the MAF sensor disconnected?

20%

Go to DTC P0101

Go to Step 5

5

Inspect for the following conditions:

    • Inspect for a collapsed air intake duct.
    • Inspect for a restricted air filter element. Refer to Air Cleaner Element Replacement .
    • Inspect the MAF sensor for foreign objects. Refer to Mass Airflow Sensor/Intake Air Temperature Sensor Replacement .
    • Inspect for restricted exhaust system. Refer to Symptoms - Engine Exhaust in Engine Exhaust.
    • Inspect for excessive fuel in the crankcase. Change the oil as necessary.
    • Inspect for rich injectors. Refer to Fuel Injector Solenoid Coil Test
    • Inspect for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect the engine control grounds for being clean, tight, and in the correct locations.
    • Inspect for an engine mechanical condition. Refer to Symptoms - Engine Mechanical in Engine Mechanical - 3.6L (LY7).

Did you find and correct the condition?

--

Go to Step 6

Go to Fuel System Diagnosis

6
  1. Turn OFF the ignition for 30 seconds.
  2. Start the engine.
  3. Reset the fuel trim system with a scan tool.
  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 7

7

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 in Vehicle DTC Information

System OK