For 1990-2009 cars

Table 1: | Fuel Trim Cells
Table 2: | DTC P0172 - Fuel Trim System Rich

### 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 1 HO2S 1 signals 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 on bank 1, the PCM will set DTC P0172. The PCMs maximum authority to control long term fuel trim allows a range between -20% and +20%. The PCMs maximum authority to control short term fuel trim allows a range between -20% 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 are as follows:

#### Fuel Trim Cells

Cell | Purge ON | Purge OFF
--- | --- | ---
Idle (Cell 0) | X | --
Decel (Cell 1) | -- | --
Normal (Cell 2) | X | X
Accel (Cell 3) | X | X
High Flow (Cell 4) | X | --
L.T. Fuel Trim average less than -18 percent
S.T. Fuel Trim greater than -2 percent

The vehicle may have to be operated in all of the above conditions 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 VSS, EST, CKP, CAM, TP, Misfire, Fuel Injector, MAF, O2, MAP, EGR, EVAP, ECT or IAT DTCs are 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 authority of -20%.

• | Short term fuel trim is at or near maximum authority of -20%.

• | 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 the MIL OFF 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 increase. However, if fuel pressure is too high, a DTC P0121 may be set.

- | Refer to Fuel System Pressure Test (VIN K) .

- | Refer to Fuel System Pressure Test (VIN 1) .

• | Fuel injector(s): 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) .

• | 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. Refer to Evaporative Emission Control System Diagnosis .

• | 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. Refer to Fuel Pressure Regulator Replacement .

| Refer 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.

• | 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

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.

#### 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 Powertrain On Board Diagnostic (OBD) System Check
2 | Are any DTCs set other than P0172? | -- | Go to Powertrain Control Module Diagnosis | 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?

-10%

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 ignition?

--

Go to Step 5

Go to Diagnostic Aids

5 | Visually and physically inspect the air filter element and replace it if necessary. Refer to Air Cleaner Element 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 screen for damage or 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.

Refer to Throttle Body Air Inlet Screen 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 the ignition switch to the OFF position.
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 Fuel System Cleaning .

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 the ignition switch to the ON position.
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. Perform the Fuel System Pressure Test. Refer to Fuel System Pressure Test (VIN K) or Fuel System Pressure Test (VIN 1) .
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 Check. 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.

Did both values change to near the specified value?

0%

Go to Step 20

Go to Step 18

18 |

1. Remove and visually/physically inspect the HO2S 1 for silicon contamination. This will be indicated by abnormal discoloration on the portion of the HO2S that is exposed to the exhaust stream.
2. If contamination is evident on the HO2S, replace the contaminated sensor. Refer to Heated Oxygen Sensor 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 action complete? | -- | Go to Step 21 | --
20 | Replace the MAF sensor. Refer to Mass Airflow 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