GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2008 🢒 SLS 🢒 DTC 🢒 DTC P2177 or P2179

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 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 rough adjustments in order to maintain an air/fuel ratio of 14.7:1. If the ECM detects an excessively lean condition, this DTC sets.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC P2177 Fuel Trim System Lean at Cruise or Accel Bank 1
    • DTC P2179 Fuel Trim System Lean at Cruise or Accel Bank 2

Conditions for Running the DTC
    • Before the ECM can report DTC P2177 or P2179 failed, DTCs P0101, P0121, P0122, P0123, P0133, P0153, P0221, P0222, P0223, P0336, P0338, P0443, P0458, P0459, P0461, P0462, P0463, P2066, P2067, and P2068 must run and pass.
    • The fuel system is in closed loop.
    • 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 amount of air flow into the engine is more than 700 grams, and the fuel level is set to non-zero.
    • The mass air flow id between 7.5-83.3 g/s.
    • The engine load is between 17-45 percent.
    • The engine speed is 1,200-3,400 RMP.
    • Throttle angle is no more than 99.6 percent.
    • Front HO2S 1 is ready.
    • Front HO2S 2 is ready.
    • The ECM commands the Lambda between 0.83-1.2.
    • The catalytic converter heating is not set.
    • The misfire rate test is not set.
    • The decel fuel cut-off is not set.
    • The wide-open throttle (WOT) is not set.
    • DTC P2177 and P2179 runs continuously once the above conditions are met for at least 300 seconds.

Conditions for Setting the DTC
    • The air/fuel ratio change modification is more than the specified value.
    • 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) at the beginning of the fourth ignition cycle, after 3 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 and related Freeze Frame data clears after 40 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 system delivery condition causes this DTC to set. Thoroughly inspect all items that cause a lean condition.
    • Any un-metered air into the engine causes this DTC to set. Thoroughly inspect all areas of the engine for vacuum leaks.
    • 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 if there is a current condition.

  2. If DTC P2177 and P2179 is set at the same time, this indicates that both banks of the engine are operating lean. Inspect for items that would cause both banks of the engine to operate lean.

    3.

  3. Disconnecting the mass air flow (MAF) sensor determines if the MAF sensor signal is skewed. If the Short Fuel Term 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.

  4. A vacuum leak causes DTC P2177 and P2179 to set at the same time. Inspect all areas of the engine for a vacuum leak. Also inspect the PCV valve for being the correct one for this application. Make sure that the engine oil fill cap is in place and that it is tight. Verify that the engine oil dip stick is fully seated.

DTC P2177 or P2179

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check - Vehicle?

--

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Important: If any DTCs are set, except P2177 or P2179 , 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 Average parameter for bank 1 or bank 2 with a scan tool.

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

23%

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

Are both banks of the engine operating lean?

--

Go to Step 5

Go to Step 7

5

  1. Start the engine.

    2. Important: Additional DTCs set with the MAF sensor disconnected.

  2. Disconnect the MAF sensor harness connector while the engine is operating.
  3. Observe the Short Term Fuel Trim parameter for bank 1 and bank 2 with a scan tool.
  4. Connect the MAF sensor after completing this step.

Does the Short Term Fuel Trim parameter for both banks of the engine change more than the specified value with the MAF sensor disconnected?

20%

Go to DTC P0101

Go to Step 6

6

Inspect for the following conditions:

    •  Splits, kinks, and proper connections of the vacuum hoses--Refer to Emission Hose Routing Diagram .
    • The throttle body and the intake manifold for vacuum leaks
    • The crankcase ventilation valve and system for leaks
    • The air intake system after the MAF sensor for vacuum leaks
    • Contaminated fuel--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Lean injectors--Refer to Fuel Injector Solenoid Coil Test .
    • The engine control grounds for being clean, tight, and in the correct locations
    • A high engine oil level condition--A high engine oil level causes oil residue to form on the MAF sensor, causing a lean indication. The MAF sensor does not need to be replaced.
    • An engine mechanical condition--Refer to Symptoms - Engine Mechanical .

Did you find and correct the condition?

--

Go to Step 8

Go to Fuel System Diagnosis

7

Inspect for the following conditions:

    • Vacuum leaks that only affect one bank of the engine--For example, the intake manifold, the injector O-rings.
    • Lean injectors--Refer to Fuel Injector Solenoid Coil Test .
    • Exhaust leaks, missing or loose exhaust hardware--Refer to Symptoms - Engine Exhaust .
    • The heated oxygen sensor (HO2S) is installed securely and the electrical connector is not contacting the exhaust system.
    • An engine mechanical condition--Refer to Symptoms - Engine Mechanical .

Did you find and correct the condition?

--

Go to Step 8

Go to Diagnostic Aids

8
  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 9

9

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

System OK