For 1990-2009 cars

### 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 voltages. 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 PCMs maximum authority to control long term fuel trim allows a range between -29% and +22%. The PCMs maximum authority to control short term fuel trim allows a range between -27% and +27%. 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 are as follows:

Cell | Purge on | Purge off
--- | --- | ---
Idle (Cell 0) | X | --
Decel (Cell 1) | X | --
Normal (Cell 2) | X | --
Accel (Cell 3) | -- | --
High Flow (Cell 4) | -- | --
L.T. Fuel Trim average less than -29%
S.T. Fuel Trim less than -27%

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

### Conditions for Running the DTC

• | No MAF, ECT, TP, MAP, CMP, CKP, IAT sensor, EVAP, EGR, HO2S, Misfire, Injector circuit, Idle speed, VSS, IC circuit DTCs set.

• | Engine coolant temperature between 20°C (68°F) and 110°C (230°F).

• | Intake air temperature is between -18°C (0°F) and 70°C (158°F).

• | Manifold absolute pressure is between 15 kPa and 85 kPa.

• | Throttle angle is steady and less than 90%.

• | Vehicle speed is less than 132 km/h (82 mph).

• | Engine speed is between 600 and 4000 RPM.

• | Baro is greater than 70 kPa.

• | Air flow is between 3 gm/s and 150 gm/s.

### Conditions for Setting the DTC

• | Long term fuel trim is at or near maximum average of 20%.

• | Short term fuel trim is at or near maximum authority of 27%.

• | All conditions have been met in the Fuel Trim Cells.

### 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 high - The system will go rich if pressure is too high. The PCM can compensate for some increase. However, if fuel pressure is too high, a DTC P0172 may be set. Refer to Fuel System Diagnosis .

• | Fuel injector(s) faulty - Refer to Fuel Injector Balance Test .

• | Check the EVAP Canister for Fuel Saturation. If the EVAP Canister is full of fuel, check canister control and hoses.

• | Disconnect the MAF sensor and see if rich condition is corrected. If so, replace the MAF sensor. Refer to MAF Sensor Replacement .

• | Check for a leaking fuel pressure regulator diaphragm by checking the vacuum line to the fuel pressure regulator for the presence of fuel.

• | An intermittent TP sensor output will cause the system to go rich due to a false indication of the engine accelerating.

• | Poor connection at the PCM - Inspect harness connectors for the following conditions:

- | Backed out terminals. Refer to Connector Repairs .

- | Improper mating. Refer to Connector Repairs .

- | Broken locks. Refer to Connector Repairs .

- | Improperly formed or damaged terminals. Refer to Connector Repairs .

- | Poor terminal to wire connection. Refer to Connector Repairs .

• | Damaged harness - Inspect the wiring harness for damage. If the harness appears to be OK, observe the affected HO2S 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 fault. Refer to Wiring Repairs .

| .

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

### Test Description

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

1. DTCs other than P0172 may indicate a condition present which may cause a rich 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.

Step | Action | Values | 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? | -- | Go to Applicable DTCs | Go to Step 3
3 |

1. Start and run the vehicle in closed loop.
2. Observe Long Term FT and Short Term FT display on the scan tool.

Are the displayed values more negative than the specified value?

-22%

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 in all cells, 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 ignition?

--

Go to Step 5

Go to Diagnostic Aids

5 | Visually and physically inspect for debris in the Air Cleaner Assembly. Inspect the air filter element and replace it if necessary. Refer to Air Cleaner Assembly Replacement . 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 for damage, or the MAF Sensor for the presence of foreign objects which may alter the airflow sample through the MAF sensor.
2. If a problem is found, repair as necessary.

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 |

1. With the engine idling, observe the Idle Air Control display on the scan tool.
2. Compare Idle Air Control display on the scan tool to Engine Scan Tool Data List . Refer to Engine Scan Tool Data List .

Is the Idle Air Control value less than typical Engine Scan Tool Data List ?

--

Go to Step 11

Go to Step 10

10 |

1. Turn the ignition OFF.
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.

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 Diagnosis .
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.

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. 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 15

15 |

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

Was a problem found?

--

Go to Step 21

Go to Step 16

16 |

1. Check fuel injector operation. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) or Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Outside 10-35 Degrees C (50-95 Degrees F) .
2. If a problem is found, repair as necessary.

Was a problem found?

--

Go to Step 21

Go to Step 17

17 |

1. Disconnect the MAF sensor electrical connector.
2. Operate the vehicle in closed loop while monitoring the Long Term FT and Short Term FT display on the scan tool.
3. Compare Long Term FT and Short Term FT display on the scan tool, to the Engine Scan Tool Data List . Refer to Engine Scan Tool Data List .

Did both values change to near the Engine Scan Tool Data List ?

0%

Go to Step 20

Go to Step 18

18 |

1. Remove and visually/physically inspect the HO2S 1 for silicon contamination. Refer to

   Notice: Contamination of the oxygen sensor can result from the use of an inappropriate RTV sealant (not oxygen sensor safe) or excessive engine coolant or oil consumption. Remove the HO2S and visually inspect the portion of the sensor exposed to the exhaust stream in order to check for contamination. If contaminated, the portion of the sensor exposed to the exhaust stream will have a white powdery coating. Silicon contamination causes a high but false HO2S signal voltage (rich exhaust indication). The control module will then reduce the amount of fuel delivered to the engine, causing a severe driveability problem. Eliminate the source of contamination before replacing the oxygen sensor.

   .
2. If contamination is evident on the HO2S, replace the contaminated sensor. Refer to Heated Oxygen Sensor (HO2S) Replacement .

Was a problem found?

--

Go to Step 21

Go to Diagnostic Aids

19 | Replace the TP sensor. Refer to Throttle Position Sensor Replacement . Is the action complete? | -- | Go to Step 21 | --
20 | Replace the MAF sensor. Refer to MAF Sensor Replacement . Is the 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 ignition?

--

Go to Step 2

System OK