GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2005 🢒 DeVille 🢒 Engine 🢒 Engine Controls - 4.6L 🢒 DTC P0172 or P0175

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 or P0175 sets.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC P0172 Fuel Trim System Rich Bank 1
    • DTC P0175 Fuel Trim System Rich Bank 2

Conditions for Running the DTC
    • DTCs P0030, P0036, P0050, P0056, P0101, P0102, P0103, P0107, P0108, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0142, P0150-P0158, P0160, P0161, P0201, P0202, P0203, P0204, P0205, P0206, P0207, P0208, P0300, P0401, P0403, P0404, P0405, P0410, P0442, P0443, P0446, P0449, P0452, P0453, P0455, P0496, P0506, P0507, P1133, P1134, P1153, P1154, P1404, are not set.
    • The engine coolant temperature (ECT) is between -40 to +127°C (-40 to +261°F).
    • The intake air temperature (IAT) is more than -7°C (+19.4°F).
    • The manifold absolute pressure (MAP) is more than 5 kPa (0.73  psi).
    • The vehicle speed is less than 134 km/h (83 mph).
    • The engine speed is between 400-6,000 RPM.
    • The barometric pressure (BARO) is more than 74 kPa (10.7 psi).
    • The mass air flow (MAF) is more than 0.5 g/s.
    • The fuel level is more than 10 percent.
    • DTC P0712 and P0175 run continuously once the above conditions have been met.
    • The engine is in Closed Loop fueling.

Conditions for Setting the DTC
    • The average long term fuel trim value is below -22 percent.
    • The above condition is present for 40 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) 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 affect fuel trim.
    • A bad mass air flow sensor can cause a rich condition and set this DTC. Refer to DTC P0101 .
    • Using scan tool, review Failure Records. If intermittent condition is suspected , refer to Intermittent Conditions .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. An EVAP canister that is saturated will cause a rich condition. Fuel in the vacuum line to the fuel pressure regulator indicates a bad regulator. If no problems are found, 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 in Vehicle DTC Information

2

Important: If any DTCs other than P0172 are set, refer to those DTCs before continuing.

  1. Install 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 ignition, with engine OFF.
  6. Review the Freeze Frame Records/Failure Records and record displayed data for this DTC.

Does the scan tool indicate that the Long Term Fuel Trim is less than the specified value?

-22%

Go to Step 3

Go to Diagnostic Aids

3
  1. Operate engine at idle.
  2. Observe HO2S parameters with a scan tool.

Are the HO2S parameters within the specified range and fluctuating?

200-800 mV

Go to Step 4

Go to Step 5

4
  1. Turn OFF engine.
  2. Visually and physically inspect the following conditions:
    3. • The inlet screen of the mass air flow (MAF) sensor for blockage
    • The vacuum hoses for splits, restrictions, and improper connections--Refer to Emission Hose Routing Diagram .
    • The air intake duct for being collapsed or restricted
    • The air filter for being dirty or restricted
    • Any objects blocking throttle body

Did you find and correct the condition?

--

Go to Step 7

Go to Step 6

5
  1. Turn OFF engine.
  2. Check the heated oxygen sensor (HO2S) for proper installation.
  3. Check to ensure electrical connectors and wires are secured and not contacting the exhaust system.

Did you find and correct the condition?

--

Go to Step 7

Go to Step 6

6

Inspect for the following conditions:

    • Excessive fuel in the crankcase
    • The evaporative emission control system--Refer to Service Bay Test .
    • Proper operation 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 7

Go to Fuel System Diagnosis

7

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

  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 8

8

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