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For 1990-2009 cars only
Makes 🢒 Buick 🢒 2008 🢒 Allure 🢒 Diagnostic Navigation 🢒 Vehicle Diagnostic Information 🢒 DTC P2178

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    •  Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC P2178: Fuel Trim System Rich at Cruise or Accel

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.

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, P0201-P0206, P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0273, P0274, P0276, P0277, P0300-P0306, P0461, P0462, P0463, 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 greater than 60°C (140°F).
    • The intake air temperature (IAT) is less than 66°C (151°F).
    • The mass air flow is between 7-83 g/s.
    • The engine load is between 16-46 percent.
    • The evaporative emission (EVAP) canister purge solenoid valve is not enabled.
    • The intake air temperature (IAT) is less than 66°C (151°F).
    • The fuel level is greater than 10 percent.
    • 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, or for a cumulative of 50 seconds.

Action Taken when the DTC Sets

DTC P2178 is a Type B DTC.

Conditions for Clearing the MIL/DTC

DTC P2178 is a Type B DTC.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Diagnostic Aids

A fuel system delivery condition causes this DTC to set. Thoroughly inspect all items that cause a rich condition.

Circuit/System Verification
  1. Ignition ON, observe the scan tool DTC information. Verify there are no other DTCs set.
    2. ⇒If there are other DTCs set refer to Diagnostic Trouble Code (DTC) List - Vehicle for further diagnosis.
  2. Engine at normal operating temperature, observe the scan tool Total Fuel Trim Avg. parameter. The parameter should display between -22 to +23 percent.
    3. ⇒If not within the specified range, verify that none of the following conditions exist:
    • A mass air flow (MAF) sensor signal that is skewed--If the Short Term FT parameters for both banks changes greater than 20 percent when the MAF sensor is disconnected, refer to DTC P0101 for further diagnosis.
    • 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
    • Inspect for excessive fuel in the crankcase. Change the oil as necessary
    • Rich fuel injectors--Refer to Fuel Injector Balance Test with Special Tool or Fuel Injector Balance Test with Tech 2
    • Incorrect fuel system pressure--Refer to Fuel System Diagnosis
    • Fuel that is contaminated--Refer to Alcohol/Contaminants-in-Fuel Diagnosis
    • Contaminated HO2S-Silicon
  3. Operate the vehicle within the conditions for running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records Data List.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.