For 1990-2009 cars

### Circuit Description

In order to provide the best possible combination of driveability, fuel economy, and emission control, the powertrain control module (PCM) uses a closed loop air/fuel metering system. The PCM monitors the heated oxygen sensor (HO2S) signal voltage and when in closed loop adjusts fuel delivery based on the HO2S signal voltage. Changes in fuel delivery will be indicated by the long term and the short term fuel trim values that are displayed on the scan tool. The ideal fuel trim value is around 0 percent. The PCM will add fuel when the heated oxygen sensor signal is indicating a lean condition. Additional fuel is indicated by fuel trim values that are above 0 percent. The PCM will reduce the amount of fuel delivered when a rich condition is indicated by the HO2S. Fuel trim values below 0 percent indicate a reduction in fuel. A fuel trim DTC can be set when exhaust emissions reach excessive levels because of a lean or rich condition.

### Conditions for Setting the DTC

• | Total fuel trim is less than or equal to -22 percent in 128 firing events.

• | Short term fuel trim is less than or equal to -11 percent in 128 firing events.

• | Barometric pressure is less than 75 kPa.

### 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 the diagnostic fails. This information is stored in the Freeze Frame buffer.

### Conditions for Clearing the MIL/DTC

• | The MIL turns OFF after three consecutively passing trips without a fault present.

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

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

### Diagnostic Aids

Check for any of the following conditions:

• | If any DTCs other than Fuel Trim P0171, P0172, Misfire P0300 to P0304, and EGR P0400 are set, diagnose those DTCs first.

• | Check for a restriction in the inlet air passage. A duct or inlet hose that collapses when hot or is blocked by debris can cause a 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 a DTC P0171 monitoring the fuel pressure while driving the vehicle at various road speeds may be necessary. Refer to Fuel System Diagnosis .

• | A malfunctioning MAF sensor.

• | Check for a damaged wiring 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 the connectors and 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 content readings. If contamination is found, determine the cause, and correct the condition before replacing the HO2S.

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

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

If the DTC P0172 is intermittent, driving the vehicle under the following conditions can verify whether the fault is present. Perform the scan tool Clear DTC Information function. Road test the vehicle while monitoring the DTC P0172 diagnostic on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0172 appears in the Not Ran Since Code Cleared list, the P0172 diagnostic has not yet run. When the DTC P0172 does not appear in the Not Ran Since Code Cleared list, the P0172 diagnostic has run. If the MIL is NOT ILLUMINATED and there is no PENDING DTC Status in DTC Information, the P0172 diagnostic has passed. DTCs MUST BE CLEARED in order to view the CURRENT STATUS of the Not Ran Since Code Cleared list. DO NOT FORGET that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for CURRENT or PENDING status, in order to determine the outcome of the diagnostic test involved.

If you cannot duplicate a DTC P0172, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was 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 taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. This step checks whether the fault is still present.

3. This step checks for a Total Fuel Trim value that is a negative number that is numerically higher than -22 percent, such as -23 percent, or -24 percent. Total Fuel Trim is a combined value of Short Term FT and Long Term FT.

4. If the condition is still present, any one of these sensors could cause this DTC to set. Check the performance of the MAF sensor, the ECT sensor, the TP sensor and the CTP switch signal. Repair any faulty conditions.

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. Install a scan tool.
3. Operate the vehicle until loop status indicates Closed Loop.
4. Observe the Short Term FT and Long Term FT parameter on the scan tool.

Is the scan tool display more than the specified value?

-11%

Go to Step 3

Go to Step 4

3 |

1. Turn ON the ignition, leaving the engine OFF.
2. Clear the scan tool information.
3. Operate the vehicle within the Freeze Frame conditions as specified.
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?

-22%

Go to Diagnostic Aids

Go to Step 4

4 | Visually and physically inspect the air filter and the air filter assembly for restrictions. Repair or replace as necessary. Was a repair necessary? | -- | Go to Step 8 | Go to Step 5
5 |

1. Turn OFF the ignition.
2. Install a fuel pressure gauge. Refer to Fuel Pressure Gage Installation and Removal .
3. Start the engine.
4. Observe the fuel pressure with the engine idling at normal operating temperature.

Is the fuel pressure within the specified value?

210-260 kPa (30-37 psi)

Go to Step 6

Go to Fuel System Diagnosis

6 |

1. Check the EVAP control system for possible over purging of fuel vapors.
2. If a problem is found, repair as necessary.

Did you find a problem?

--

Go to Step 8

Go to Step 7

7 | Important: Before replacing the oxygen sensor because of contamination, determine and correct the cause of the contamination.

1. Remove the HO2S 1.
2. Visually inspect the HO2S 1 for damage or contamination.
3. If damage or contamination is evident, replace the HO2S 1. Refer to Heated Oxygen Sensor Replacement .

Did the HO2S 1 require replacement?

--

Go to Step 8

Go to Symptoms

8 |

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

Are any DTCs displayed on the scan tool?

--

Go to Diagnostic Trouble Code (DTC) List

System OK