GM Service Manual Online
For 1990-2009 cars only

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. The fuel trim diagnostic will conduct a test to determine if a rich failure actually exists or if excessive vapor from the evaporative emission (EVAP) canister is causing a rich condition. If the PCM detects an excessively rich condition, DTC P0172 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, P0496, P0506, P0507, P1404, P1516, P1518 are not set.
    • The engine is in Closed Loop status.
    • The 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 25-99 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.
    • DTC P0172 runs continuously once the above conditions have been met.

Conditions for Setting the DTC

    • The average Long Term fuel trim value is below -24 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

    • A malfunctioning MAF sensor can cause a rich condition and set this DTC.
    • If an intermittent condition is suspected , refer to Intermittent Conditions .

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

Are any DTCs other than DTC P0172 also set?

 

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

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 the normal operating temperature in Closed Loop.
  5. Observe the Long Term FT parameter.

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

-24%

Go to Step 4

Go to Diagnostic Aids

4

Observe HO2S bank 1 sensor 1 and HO2S bank 2 sensor 1 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. • The inlet screen of the mass airflow (MAF) sensor for blockage--Refer to Mass Airflow Sensor Replacement .
    • Vacuum hoses for splits, kinks, and proper connections--Refer to Emission Hose Routing Diagram .
    • The air intake duct for being collapsed or restricted--Refer to Air Cleaner Inlet Duct Replacement .
    • The air filter for being dirty or restricted--Refer to Air Cleaner Element Replacement .
    • Objects blocking the throttle body--Refer to Throttle Body Assembly Replacement .

Did you find and correct the condition?

--

Go to Step 9

Go to Step 8

6

  1. Turn OFF the engine
  2. Visually and physically inspect the following items:
  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 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

Inspect for the following:

    • Excessive fuel in the crankcase
    • Saturated evaporative canister, stuck purge valve or fuel cap not sealing
    • Excessive fuel pressure--Refer to Fuel System Diagnosis .
    • Properly functioning fuel injectors--Refer to Fuel Injector Solenoid Coil Test .
    • Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

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 - Vehicle in Vehicle DTC Information

System OK