GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2005 🢒 Century 🢒 Vehicle Control Systems 🢒 Vehicle DTC Information 🢒 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 Loop and Closed Loop. During Open Loop, the PCM determines fuel delivery based on sensor signals, without oxygen sensor input. During Closed Loop, the oxygen sensor inputs are added and used by the PCM to calculate the short and long term fuel trim, fuel delivery adjustments. If the oxygen sensors indicate a lean condition, the fuel trim values will be above 0 percent. If the oxygen sensors indicate a rich condition, the fuel trim values will be below 0 percent. The short term fuel trim values change rapidly in response to the heated oxygen sensor (HO2S) voltage signals. The long term fuel trim makes coarse adjustments in order to maintain an air/fuel ratio of 14.7:1. 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 P0101-P0103, P0107, P0108, P0121-P0123, P0130-P0135, P0137, P0138, P0140, P0141, P0201-P0206, P0300, P0401, P0403-P0405, P0412, P0418, P0442, P0446, P0455, P0496, P0506, P0507, or P1404 are not set.
    • The engine coolant temperature (ECT) is between 20-110°C (68-239°F).
    • The intake air temperature (IAT) is between 18-70°C (64-158°F).
    • The manifold absolute pressure (MAP) is between 15-105 kPa (2.1-15.2 psi).
    • The vehicle speed is less than 132 km/h (82 mph).
    • The engine speed is between 550-4,000 RPM.
    • The barometric pressure is more than 70 kPa (10.1 psi).
    • The idle purge is enabled.
    • The mass air flow (MAF) is between 2.8-150 g/s.
    • The fuel level is more than 10 percent.

Conditions for Setting the DTC
    • The average long term fuel trim value is below -19.5 percent.
    • The above condition is present for 40 seconds.
    • This diagnostic runs continuously once the above conditions have been met.

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
    • Fuel contamination, such as water and alcohol, will effect the fuel trim.
    • A malfunctioning MAF sensor can cause a rich condition and set this DTC. Refer to DTC P0101 .
    • Review the Failure Records with a scan tool. If an intermittent condition is suspected , refer to Intermittent Conditions .
    • 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 numbers below refer to the step numbers on the diagnostic table.

  1. If the conditions were not corrected, refer to Fuel System Diagnosis for a possible fuel condition.

  2. An EVAP canister that is saturated will cause a rich condition. Fuel in the vacuum line to the fuel pressure regulator indicates a leaking regulator. If the conditions were not corrected, a worn cam, 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 set?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 3

3
  1. Install a scan tool.
  2. Start and idle the engine at the normal operating temperature in Closed Loop.
  3. Record the Long Term Fuel Trim data.
  4. Turn OFF the engine.
  5. Turn ON the ignition, with the engine OFF.
  6. Review the Freeze Frame/Failure Records and record the displayed data for this DTC.

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 the HO2S 1 parameters with a scan tool.

Does the scan tool indicate that the values 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 conditions:
    3. • Blockage of the inlet screen of the MAF sensor
    • Splits or kinks in the vacuum hoses
    • Improper connections of the vacuum hoses--Refer to Emission Hose Routing Diagram .
    • Collapsed or restricted air intake duct
    • Dirty or restricted air filter
    • Objects blocking the throttle body

Did you find and correct the condition?

--

Go to Step 10

Go to Step 9

6

  1. Turn OFF the engine.
  2. Inspect the HO2S for proper installation.
  3. Ensure that the electrical connectors and the wires are secure and not contacting the exhaust system.

Did you find and correct the condition?

--

Go to Step 10

Go to Step 7

7

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 10

Go to Step 8

8

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 10

Go to Fuel System Diagnosis

9

Inspect for the following conditions:

    • Excessive fuel in the crankcase
    • Evaporative emissions of the control system--Refer to Service Bay Test .
    • Proper operations of the fuel pressure regulator--Refer to Fuel System Diagnosis .
    • Proper operation of the injectors--Refer to Fuel Injector Solenoid Coil Test .

Did you find and correct the condition?

--

Go to Step 10

Go to Symptoms - Engine Mechanical

10

Important: After any of the above repairs, use the scan tool Fuel Trim Reset function in order to reset the Long Term Fuel Trim.

  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 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 11

11

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