GM Service Manual Online
For 1990-2009 cars only

Circuit Description

To provide the best possible combination of driveability, fuel economy, and emission control, a closed loop air/fuel metering system is used. While in closed loop, the PCM monitors the HO2S 1 signal and adjusts fuel delivery based upon the HO2S signal voltage. A change made to fuel delivery will be indicated by the long and short term fuel trim values which can be monitored with a scan tool. Ideal fuel trim values are around 0%; if the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 0%. If a rich condition is detected, the fuel trim values will be below 0%, indicating that the PCM is reducing the amount of fuel delivered. If an excessively rich condition is detected, the PCM will set DTC P0172.

The PCM's maximum authority to control long term fuel trim allows a range between -23% and +16%. The PCM's maximum authority to control short term fuel trim allows a range between -11% and +20%. The PCM monitors fuel trim under various operating conditions (fuel trim cells) before determining the status of the fuel trim diagnostic. The fuel trim cells used to determine fuel trim are as follows:

    • Idle, purge commanded (Cell 0).
    • Deceleration, purge commanded (Cell 1).
    • Normal driving, purge commanded (Cell 2).
    • Deceleration, no purge commanded (Cell 6).
    • Normal driving, no purge commanded (Cell 7).

The vehicle may have to be operated in all of the above conditions before the PCM determines fuel trim status. The amount that fuel trim deviates from 0% in each cell and the amount that each cell is weighted directly affects whether or not the vehicle must be operated in all of the cells described above to complete the diagnostic.

Conditions for Setting the DTC

    • No MAF sensor, ECT sensor, HO2S, TP sensor, EVAP, EGR, Misfire, Injector circuit, Idle speed, MAP sensor, or IAT sensor DTC(s) set.
    • Engine coolant temperature between 20°C (68°F) and 110°C (230°F).
    • Intake air temperature is between -18°C (0°F) and 40°C (104°F).
    • BARO is greater than 75 kPa.
    • Manifold absolute pressure is between 18 kPa and 90 kPa.
    • Throttle angle is less than 90%.
    • Vehicle speed is less than 70 mph.
    • Mass air flow is between 9 gm/s and 180 gm/s.
    • Engine speed between 500 and 4000 RPM.
    • Long term fuel trim is at or near maximum authority of -23%.
    • Short term fuel trim is at or near maximum authority of -11%.
    • All conditions have been met in Fuel Trim Cells 0, 1, 2, 6, and/or 7.

Action Taken When the DTC Sets

    • The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.
    • The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.

Conditions for Clearing the MIL/DTC

    • The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been run and passed.
    • The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
    • The DTC can be cleared by using the scan tool.

Diagnostic Aids

Check the following items:

    • Fuel pressure. The system will go rich if pressure is too high. The PCM can compensate for some decrease. However, if fuel pressure is too high, a DTC P0172 may be set. Refer to Fuel System Pressure Test .
    • Fuel injector(s). Go to Fuel Injector Solenoid Coil Test .
    • EVAP Canister. Check the EVAP canister for fuel saturation. If the canister is full of fuel, visually and physically check the EVAP and fuel system components for damage.
    • Mass Air Flow Sensor. Disconnect the MAF sensor and see if rich condition is corrected. If so, replace the MAF sensor. Refer to Mass Airflow Sensor Replacement .
    • Fuel Pressure Regulator. Check for a leaking fuel pressure regulator diaphragm by checking vacuum line to regulator for the presence of fuel. If a problem is found, replace the fuel pressure regulator. Go to Fuel Pressure Regulator Replacement .
    • TP Sensor. An intermittent TP sensor output will cause the system to go rich due to a false indication of the engine accelerating. Throttle Angle displayed on a scan tool should increase steadily from 0% to 100% when opening the throttle. If not, check the TP sensor for loose mounting screws. If the TP mounting screws are OK, replace the TP sensor. Go to Throttle Position Sensor Replacement .
    • Poor connection at the PCM. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
    • Damaged harness. Inspect the wiring harness for damage.
    • If the harness appears to be OK, observe the HO2S 1 display on the scan tool while moving connectors and wiring harnesses related to the engine harness. A change in the display will indicate the location of the malfunction.

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

  1. DTCs other than P0172 may indicate a condition present which may cause a lean condition. If this is the case, repairing the condition which caused the other DTC will most likely correct the DTC P0172.

  2. If the DTC P0172 test passes while the Fail Records conditions are being duplicated, the rich condition is intermittent. Refer to Diagnostic Aids for additional information on diagnosing intermittent conditions.

DTC P0172 - Fuel Trim System Rich

Step

Action

Value(s)

Yes

No

1

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

--

Go to Step 2

Go to the Powertrain On Board Diagnostic (OBD) System Check

2

Are any DTCs set other than P0172?

--

Repair the other DTCs first -- Go to Powertrain Control Module Diagnosis

Go to Step 3

3

  1. Start and run the vehicle in closed loop.
  2. Observe L.T. Fuel Trim and S.T. Fuel Trim display on the scan tool.

Are the displayed values at or near the specified values?

    • L.T. Fuel Trim is the first specified value.
    • S.T. Fuel Trim is the second specified value.

-23%

-11%

Go to Step 5

Go to Step 4

4

  1. Review and record the scan tool Fail Records data.
  2. Clear the DTC P0172 and operate the vehicle to duplicate the Fail Records conditions.
  3. Monitor the scan tool Specific DTC info for DTC P0172 while operating the vehicle to duplicate the Fail Records conditions.
  4. Continue operating the vehicle until the DTC P0172 test runs and note test result.

Does the scan tool indicate DTC P0172 failed this ign?

--

Go to Step 5

Refer to Diagnostic Aids

5

Visually and physically inspect the air filter element and replace it if necessary. Refer to Air Cleaner Element .

Did the air filter require replacement?

--

Go to Step 21

Go to Step 6

6

Visually and physically inspect the air intake duct for being collapsed or restricted and repair if necessary.

Did your inspection reveal a condition requiring repair?

--

Go to Step 21

Go to Step 7

7

  1. Inspect the throttle body inlet screen for damage or for the presence of foreign objects which may partially block airflow through the screen.
  2. If a problem is found, repair as necessary. Refer to Throttle Body Assembly Replacement .

Was a problem found?

--

Go to Step 21

Go to Step 8

8

Start the engine and note the idle quality.

Is a low or unsteady idle being experienced?

--

Go to Step 9

Go to Step 11

9

With the engine idling, observe the Idle Air Control display on the scan tool.

Is Idle Air Control value less than the specified value? (Value is displayed in counts).

100

Go to Step 11

Go to Step 10

10

  1. Turn OFF the ignition switch.
  2. Physically inspect the throttle body bore, throttle plate, and IAC passages for coking and foreign objects.
  3. If a problem is found, repair as necessary. Refer to Throttle Body Assembly Replacement .

Was a problem found?

--

Go to Step 21

Go to Step 11

11

  1. Check the idle air control system. Refer to the Idle Air Control (IAC) System Check .
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 21

Go to Step 12

12

  1. Disconnect the vacuum hose from the fuel pressure regulator and inspect the hose for the presence of fuel.
  2. If fuel is present in the vacuum hose, replace the fuel pressure regulator. Refer to Fuel Pressure Regulator Replacement .

Did the fuel pressure regulator require replacement?

--

Go to Step 21

Go to Step 13

13

  1. Turn ON the ignition switch.
  2. Monitor the TP Angle display on the scan tool while slowly depressing the accelerator pedal.

Does the TP Angle display increase steadily and evenly from minimum value at closed throttle to maximum value at wide-open throttle?

0%-100%

Go to Step 14

Go to Step 19

14

  1. Disconnect the MAF sensor electrical connector.
  2. Operate the vehicle in closed loop while monitoring the L.T. Fuel Trim and S.T. Fuel Trim display on the scan tool.

Did both values change to near the specified value?

0%

Go to Step 20

Go to Step 15

15

  1. Perform the Fuel System Pressure Test. Refer to Fuel System Pressure Test .
  2. If a problem is found, repair as necessary.

Was problem found?

--

Go to Step 21

Go to Step 16

16

  1. Perform the Evaporative Emissions Control (EVAP) System Check.
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 21

Go to Step 17

17

  1. Check fuel injector operation. Refer to Fuel Injector Solenoid Coil Test .
  2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 21

Go to Step 18

18

  1. Remove and visually/physically inspect the HO2S 1 for silicon contamination. This will be indicated by abnormal discoloration of the portion of the HO2S that is exposed to the exhaust stream.
  2. If contamination is evident on the HO2S, replace contaminated sensor. Refer to Heated Oxygen Sensor Replacement .

Was a problem found?

--

Go to Step 21

Go to Diagnostic Aids

19

  1. Check the TP sensor mounting screws and tighten or replace them as necessary if they are loose or missing.
  2. If the screws are OK, replace the TP sensor. Refer to Throttle Position Sensor Replacement .

Is action complete?

--

Go to Step 21

--

20

Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement .

Is action complete?

--

Go to Step 21

--

21

  1. Review and record the scan tool Fail Records data.
  2. Clear the DTC P0172 and operate the vehicle to duplicate the Fail Records conditions.
  3. Monitor the scan tool Specific DTC info for DTC P0172 while operating the vehicle to duplicate the Fail Records conditions.
  4. Continue operating the vehicle until the DTC P0172 test runs and note test result.

Does the scan tool indicate DTC P0172 failed this ign?

--

Go to Step 2

System OK