For 1990-2009 cars

### Circuit Description

The PCM uses a closed-loop air/fuel metering system in order to provide the best combination of driveability and fuel economy with the emission controls. In the closed-loop mode, the PCM monitors the signal voltage of the heated oxygen sensor (HO2S) and adjusts the fuel delivery accordingly. The long-term and the short-term fuel trim values indicate changes in fuel delivery. The best fuel trim value is around 0 percent. The PCM will add fuel if the HO2S signal indicates a lean condition. Trim values that are above 0 percent indicate additional fuel. The PCM reduces the fuel delivery if the HO2S indicates a rich condition. Fuel trim values below 0 percent indicate a reduction in fuel. A fuel trim DTC can set if exhaust emissions reach excessive levels because of a lean condition or because of a rich condition.

### Conditions for Running the DTC

• | The barometric pressure is more than 75 kPa.

• | The fuel tank level is more than 25 percent.

• | The intake air temperature is between -10°C and +70°C (14°F-158°F).

• | The engine coolant temperature is above 80°C (176°F).

• | The engine is running at a steady speed.

• | The engine is operating in the closed-loop mode.

### Conditions for Setting the DTC

• | The total fuel trim is less than or equal to -30 percent for 5 seconds.

• | The short term fuel trim is less than or equal to -20 percent for 45 seconds.

### Action Taken When the DTC Sets

• | The PCM illuminates the malfunction indicator lamp (MIL) after two consecutive ignition cycles in which the diagnostic runs with the fault active.

• | The PCM records the operating conditions at the time that the diagnostic fails. This information is stored in the Freeze Frame buffer.

### Conditions for Clearing the DTC

• | The MIL turns OFF after 3 consecutive trips during which the diagnostic runs and does not detect a fault.

• | A History DTC clears after 40 consecutive warm-up cycles without a fault.

• | Use the Clear DTC Information function of the scan tool, or disconnect the PCM battery feed in order to clear the DTC.

### Diagnostic Aids

Check for any of the following conditions:

• | Check for a restriction in the inlet air passage. A duct or an inlet hose that collapses when hot, or a hose that is blocked by debris can cause DTC P0172 to set.

• | Check for the correct fuel pressure. The fuel system will be rich if fuel pressure is too high. In order to determine the cause of the DTC, monitor the fuel pressure while driving the vehicle at various road speeds. Refer to Fuel System Diagnosis .

• | A MAP sensor output that causes the PCM to sense a higher-than-normal manifold pressure can cause the system to go rich. Disconnect the MAP sensor in order to allow the PCM to substitute a fixed default value for the MAP. If the rich condition disappears when the sensor is disconnected, substitute a known good MAP sensor, and retest the system.

• | Inspect the wiring harness for damage. If the harness appears OK, monitor the HO2S 1 with the scan tool while moving the connectors and the wiring harnesses related to the sensor. A change in the display will indicate the location of the fault.

• | Check for a contaminated oxygen sensor. Silicone and other contaminants can cause the oxygen sensor to provide false exhaust oxygen readings. Before replacing the HO2S, determine and correct the cause of any contamination.

• | Inspect the HO2S 1 for water intrusion into the wiring harness and into the sensor housing. Water can create a short to voltage in the HO2S signal circuit, and cause a false rich indication.

• | Check the HO2S 1 and the MAP sensor for a faulty electrical connection to the PCM.

If DTC P0172 cannot be duplicated, the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.

### Test Description

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

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and to store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data that was taken when the fault occurred. The scan tool stores the information for later reference.

2. This step determines if the fault is present.

3. This step checks for excess fuel from the fuel injectors. The fault may be a fuel injector or the fuel injector electrical circuit.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check
2 |

1. Start the engine.
2. Reach normal operating temperature.
3. Operate the vehicle until the scan tool indicates Closed Loop.
4. Observe the Short Term FT parameter on the scan tool.

Is the scan tool display more than the specified value?

-20 %

Go to Step 3

Go to Step 4

3 |

1. Turn ON the ignition, leaving the engine OFF.
2. Perform the clear DTC information function on the scan tool.
3. Operate the vehicle within the Freeze Frame conditions as noted.
4. Observe the Total Fuel Trim parameter on the scan tool.

Was the scan tool display more than the specified value while operating the vehicle within the Freeze Frame conditions?

-30 %

Go to Diagnostic Aids

Go to Step 4

4 |

1. Visually and physically inspect the air filter and the air filter assembly for restrictions.
2. Repair as necessary.

Was a repair necessary?

--

Go to Step 10

Go to Step 5

5 |

1. Turn OFF the ignition.
2. Install a J 34730-1A fuel pressure gauge or an equivalent. Refer to Fuel Pressure Gage Installation and Removal .
3. Start the engine.
4. Observe the fuel pressure with the engine idling at normal operating temperatures.

Is the fuel pressure within the specified range?

200-240 kPa (28.5-35 psi)

Go to Step 6

Go to Fuel System Diagnosis

6 |

1. Check the EVAP control system for the possible over purging of fuel vapors. Refer to Evaporative Emission Control System Diagnosis .
2. Repair any problems that were found in the EVAP control system.

Was a problem found and repaired?

--

Go to Step 10

Go to Step 7

7 |

1. Remove the HO2S 1.
2. Visually inspect the HO2S 1 for damage or for contamination.

Important: Before replacing a contaminated HO2S, repair the cause of the contamination.

3. Replace the HO2S 1 if any damage or any contamination is evident. Refer to Heated Oxygen Sensor Replacement to .

Did the HO2S 1 require replacement?

--

Go to Step 10

Go to Step 8

8 | Check the fuel injectors for any of the following conditions:

• | The discharge of fuel is regular and even. Refer to Sequential Multiport Fuel Injector Balance Test .

• | Fuel does not leak from the ball valve of the injector.

• | The fuel is sprayed in a conical pattern.

• | The electrical resistance of the injector coils are all within specifications. Refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Between 10-35 Degrees C (50-95 Degrees F) .

Did the fuel injectors pass all the checks?

--

Go to Diagnostic Aids

Go to Step 9

9 | Replace any faulty fuel injectors. Refer to Fuel Injector and Fuel Rail Replacement . Is the action complete? | -- | Go to Step 10 | --
10 |

1. Perform the Clear DTC Information function on the scan tool, and road-test the vehicle within the Freeze Frame conditions which set the DTC.
2. Review the scan tool data, and check the data for DTCs. The repair is complete if no DTCs are stored.

Are any DTCs displayed on the scan tool?

--

Go to the applicable DTC table

System OK