For 1990-2009 cars

### Circuit Description

The closed-loop air/fuel metering system that is used by the PCM provides the best possible combination of driveability and fuel economy with emission control. When in closed-loop, the PCM monitors signal voltage of the the heated oxygen sensor (HO2S) and adjusts the fuel delivery accordingly. The long-term and the short-term fuel trim values that the scan tool displays indicate changes in fuel delivery. The best fuel trim value is around 0 percent. The PCM will add fuel if the HO2S signal is indicating a lean condition. Fuel trim values that are above 0 percent indicate addition 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. If exhaust emissions reach excessive levels because of a lean condition or because of a rich condition, a fuel trim DTC may set.

### Conditions for Running the DTC

• | The barometric pressure 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 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 more than or equal to 30 percent for 5 seconds.

• | The short term fuel trim is more 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 three 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 vacuum leaks.

• | Check for exhaust leaks.

• | Check for the correct fuel pressure. The fuel system will be lean if fuel pressure is too low. Monitor the fuel pressure while driving the vehicle at various road speeds in order to determine the cause of DTC P0171. Refer to Fuel System Diagnosis .

• | Check for fuel contamination. Small amounts of water can be delivered to the fuel injectors and cause a lean exhaust indication. A lean exhaust indication can also be caused by too much alcohol in the fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

• | A MAP sensor output that causes the PCM to sense a low manifold pressure can cause the system to go lean. Disconnecting the MAP sensor allows the PCM to substitute a fixed default value for the MAP sensor. If the lean condition disappears with the MAP sensor disconnected, substitute a known good MAP sensor, and retest the system.

• | Check for a damaged wiring harness. 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 lean exhaust readings. Before replacing the HO2S, determine the cause of contamination, and correct the condition.

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

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

If DTC P0171 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 on the diagnostic table.

1. The Powertrain OBD System Check prompts the technician to complete some basic checks and 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 determines that the fault is present.

3. Engine control sensors that are found to be out range (skewed) or fixed, can cause the engine to run lean. Refer to the applicable sensor DTC table for direction in diagnosing any possible sensor error.

4. This step checks for inadequate fuel delivery from a fuel injector. 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 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 less 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 scan tool Clear DTC Information function.
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 less than the specified value while operating the vehicle within the Freeze Frame conditions as specified?

30%

Go to Diagnostic Aids

Go to Step 4

4 |

1. Visually and physically check for any of the following conditions:

• | Vacuum hoses for splits, kinks and improper connections.

• | PCV valve for proper operation.

• | Exhaust system for leaks near the HO2S.

• | EVAP control system for proper operation. Refer to Evaporative Emission Control System Diagnosis .

2. Repair as necessary.

Did any of the above checks isolate a condition requiring a repair?

--

Go to Step 11

Go to Step 5

5 |

1. Start the engine.
2. Run the engine to normal operating temperatures.
3. Observe the operation of the following sensors at idle, with a scan tool.

• | The MAP sensor

• | The ECT sensor

• | The TP snesor

4. Compare the observed sensor readings with the TYPICAL SCAN TOOL DATA VALUES located in the engine scan tool data list. Refer to Engine Scan Tool Data List .
5. Inspect the vehicle for the cause of any errant sensor readings and repair as necessary.

Was a repair necessary?

--

Go to Step 11

Go to Step 6

6 |

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 value?

200-240 kPa (28.5-35 psi)

Go to Step 7

Go to Fuel System Diagnosis

7 | Check the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Was the fuel contaminated? | -- | Go to Step 8 | Go to Step 9
8 | Replace any contaminated fuel. Refer to Fuel System Cleaning . Is the action complete? | -- | Go to Step 11 | --
9 | 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 .

• | The fuel is dispersed in a conical spray 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 10

10 | Replace any faulty fuel injectors. Refer to Fuel Injector and Fuel Rail Replacement . Is the action complete? | -- | Go to Step 11 | --
11 |

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 and check 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