GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Malibu 🢒 DTC 🢒 DTC P0172

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 optimum air/fuel ratio. A block of memory cells contains 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.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0172 Fuel Trim System Rich Bank 1

Conditions for Running the DTC
    • DTCs P0030, P0036, P0053, P0054, P0068, P0101, P0102, P0103, P0107, P0108, P0112, P0113, P0120, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0201, P0202, P0203, P0204, P0205, P0206, P0220, P0300, P0401, P0403, P0404, P0405, P0406, P0420, P0442, P0446, P0449, P0451, P0452, P0453, P0454, P0455, P0462, P0463, P0464, P0496, P0506, P0507, P1133, P1134, P1404, P1516, P2101, P2107, P2108, P2119, P2120, P2125, P2135, P2138, P2A00, P2A01 are not set.
    • The engine is in Closed Loop status.
    • The Fuel Trim Learn is enabled.
    • The engine coolant is between -40 and +132°C (-40 and +270°F).
    • The intake air temperature (IAT) is between -21 and +150°C (-6 and +302°F).
    • The manifold absolute pressure (MAP) is between 5-105 kPa (0.7-15.2 psi).
    • The vehicle speed is between 13-132 km/h (8-82 mph).
    • The engine speed is between 500-6,000 RPM.
    • The barometric pressure (BARO) is more than 70 kPa (10.1 psi).
    • The mass air flow (MAF) is between 0.5-510 g/s.
    • The scan tool special functions are not active.

Conditions for Setting the DTC
    • The average Long Term fuel trim value is less than a calibrated value.
    • The above condition is present for more than 2 minutes.
    • This diagnostic runs continuously.

Action Taken When the DTC Sets

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 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. The control module illuminates the malfunction indicator lamp (MIL) when one of the following occur:

    • The control module detects the same fuel trim failure during 2 consecutive trips.
    • The control module detects any fuel trim failure during any subsequent trip if the conditions at the time of failure meet the following criteria:
       - The engine load is within 20 percent of the previous test that failed.
       - The engine speed is within 375 RPM of the previous test that failed.
       - The engine coolant temperature is in the same range of the previous test that failed.

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 malfunctioning MAF sensor can cause a rich condition and set this DTC.
    • If an intermittent condition is suspected , refer to Testing for Intermittent Conditions and Poor Connections .
    • This DTC may set due to poor idle characteristics. A possible cause may be uncontrolled fueling due to an open or high resistance in the HO2S 1 low signal circuit. Before replacing any component, ensure that this condition does not exist.

Test Description

The number below refers to the step number on the diagnostic table.

  1. If conditions were not corrected, a worn CAM, a worn intake, or exhaust valves or other engine mechanical failure may be at fault.

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

Are any DTCs other than DTC P0172 also set?

 

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 3

3

Important: Disregard any transmission symptoms, antilock brake system (ABS) indicators, and traction control system (TCS) indicators until any fuel trim faults are repaired. A fuel trim fault may cause default actions such as harsh shifts and illumination of the ABS/TCS indicators.

  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?

-19.5%

Go to Step 4

Go to Diagnostic Aids

4
  1. Operate the engine at idle.
  2. 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 11

Go to Step 8

6
  1. Turn OFF the engine.
  2. Turn ON the ignition, with the engine OFF.
  3. Observe the manifold absolute pressure (MAP) sensor parameter with a scan tool. Refer to Altitude Versus Barometric Pressure .
  4. The MAP sensor parameter should be within the range specified for your altitude.

Does the MAP indicate the correct barometric pressure?

--

Go to Step 7

Go to Manifold Absolute Pressure Sensor Diagnosis

7
  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 11

Go to Step 8

8

Test for a short between the signal circuit and the low reference circuit or ground. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 11

Go to Step 9

9

Test the HO2S 1 low signal circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

--

Go to Step 11

Go to Fuel System Diagnosis

10

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 11

Go to Symptoms - Engine Mechanical

11

Important: Reset the Long Term Fuel Trim with the scan tool Fuel Trim Reset function after any of the above repairs.

  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 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 12

12

In order to ensure that the performance of the catalyst has not been affected by the fault that set this DTC, operate the vehicle within the Conditions for running and setting DTC P0420 or P0430 and verify that DTCs P0420 and P0430 run and pass. Refer to DTC P0420 or DTC P0430 .

Does the DTC run and pass?

--

Go to Step 13

Go to DTC P0420 or DTC P0430

13

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