For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptors

DTC P0171: Fuel Trim System Lean
DTC P0172: Fuel Trim System Rich

### Circuit Description

The engine control module (ECM) controls a Closed Loop air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The ECM monitors the heated oxygen sensor (HO2S) signal voltage and adjusts the fuel delivery based on the signal voltage while in Closed Loop. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes fine tune the engine fueling. The long term fuel trim values change in response to trends in the short term fuel trim. The long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. The ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the ECM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition. The fuel trim cells are based on the engine speed and load.

### Conditions for Running the DTC

• | DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0201, P0202, P0203, P0204, P0300, P0301, P0302, P0303, P0304, P0442, P0443, P0449, P0451, P0452, P0453, P0454, P0455, P0461, P0462, P0463, P0464, P0496, P0506, and P0507 are not set.

• | The fuel system is in Closed Loop.

• | The engine speed is between 400-6,100 RPM

• | The engine coolant temperature (ECT) is between -38 to +130°C (-36 to 266°F).

• | The intake air temperature (IAT) sensor is between -20°C to +150°C (-4°F to +302°F).

• | The manifold absolute pressure (MAP) is between 15-255 kPa (2-37 psi).

• | The barometric pressure (BARO) is greater than 70 kPa (10 psi).

• | The engine airflow is between 1-512 g/s.

• | The vehicle speed is less than 140 km/h (87 mph).

• | The fuel level is greater than 10 percent.

• | The DTCs run continuously when the above conditions are met.

### Conditions for Setting the DTC

The average long term FT weighted average value is greater or less than a calibrated value.

### Action Taken When the DTC Sets

DTCs P0171 and P0172 are Type B DTCs.

### Conditions for Clearing the MIL/DTC

DTCs P0171 and P0172 are Type B DTCs.

### Diagnostic Aids

• | The system will become lean if a fuel injector is not supplying enough fuel - P0171.

• | A lean condition could be present during high fuel demand due to a fuel pump that does not deliver enough fuel, a plugged fuel filter, or a restricted fuel pipe - P0171.

• | The system will become rich if a fuel injector is supplying too much fuel - P0172.

• | A MAF sensor that is installed incorrectly may set this DTC.

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### Connector End View Reference

Component Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

1. Engine running, observe the DTC information with a scan tool. Verify there are no HO2S DTCs set.

⇒ | If any DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle.

2. Determine the current vehicle testing altitude. Compare the MAP sensor pressure parameter to the reading in the Altitude Versus Barometric Pressure table. The MAP sensor pressure parameter should be within the specified range indicated in the table.

⇒ | If the MAP sensor pressure is not within the range specified for your altitude, refer to DTC P0106.

3. Engine at normal operating temperature, observe the scan tool Long Term FT parameter. The reading should be between -5 to +5 percent.

⇒ | If not within the specified range, verify none of the following conditions exist:

#### P0171

• | Splits, kinks, or improper connections at the vacuum hoses

• | Vacuum leaks at the intake manifold, throttle body, and injector O-rings

• | Leaking crankcase ventilation system--Refer to Crankcase Ventilation System Inspection/Diagnosis.

• | Missing, restricted, or leaking exhaust components--Refer to Symptoms - Engine Exhaust.

• | The fuel system is operating lean. Refer to Fuel System Diagnosis.

• | The fuel injectors are operating lean. Refer to Fuel Injector Balance Test with Special Tool.

• | Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis.

#### P0172

• | Restricted air filter

• | The air intake duct for being collapsed or restricted.

• | Excessive fuel in the crankcase

• | The incorrect operation of the evaporative emission control system

• | Missing, restricted, or leaking exhaust components--Refer to Symptoms - Engine Exhaust.

• | The fuel system is operating rich. Refer to Fuel System Diagnosis.

• | The fuel injectors are operating rich. Refer to Fuel Injector Balance Test with Special Tool.

• | Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis.

4. If all conditions test normal, test the engine for a mechanical condition. Refer to Symptoms - Engine Mechanical.
5. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.