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 P0016, P0106, P0107, P0108, P0112, P0113, P0115, P0116, P0117, P0118, P0122, P0123, P0131, P0132, P0133, P0134, P0135, P0201, P0202, P0203, P0204, P0217, P0300, P0335, P0336, P0340, P0351, P0352, P0402, P0404, P0405, P0406, P042E, P0443, P0506, P0507, P1134, and P2297 are not set.

• | The fuel system is in closed loop.

• | The engine speed is between 700-6,000 RPM

• | The engine coolant temperature is between 70-115°C (158-239°F).

• | The intake air temperature (IAT) sensor is between -40 to +120°C (-40 to +248°F).

• | The throttle position is between 0-95 percent.

• | The manifold absolute pressure (MAP) is between 25-99.7 kPa.

• | The barometric pressure (BARO) is more than 72 kPa.

• | The engine airflow is between 1.5-45 g/s.

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

• | The ignition 1 voltage is greater than 11 volts.

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

### Conditions for Setting the DTC

#### P0171

The average long term fuel trim cell values are more than 30 percent for 10 seconds.

#### P0172

The average long term fuel trim cell values are less than -30 percent for 10 seconds.

### 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

#### DTC P0171

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

• | 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.

• | Review the Freeze Frame/Failure Records with a scan tool.

#### DTC P0172

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

• | Review the Freeze Frame/Failure Records with a scan tool.

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### Connector End View Reference

• | Engine Control Module Connector End Views

• | Engine Controls 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

• | Scan Tool Data List

• | Scan Tool Data Definitions

• | Scan Tool Output Controls

### Circuit/System Verification

1. Verify that other DTCs are not set.

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

2. Ignition ON, observe the manifold absolute pressure (MAP) sensor pressure parameter and compare the reading on the scan tool to the barometric pressure table. Refer to Altitude Versus Barometric Pressure .

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

3. Engine running, observe the TP sensor percentage parameter. The reading should be less than 10 percent.

⇒ | If greater than 10 percent, refer to DTC P0122 or P0123 .

4. Engine running, observe the HO2S sensor voltage parameter. The reading should fluctuate within 200-800 mV.

⇒ | If out of the specified range, refer to DTC P0131 or P0132 and DTC P0133, P1133, or P1134 .

5. With the engine at normal operating temperature, observe the Long Term FT. The Long Term FT parameter should be between -5 to +5 percent.
6. If the vehicle passes the Circuit/System Verification Test, then operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records data list.

### Circuit/System Testing

#### DTC P0171

1. Verify that none of the following conditions exist:

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

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

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

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

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

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

• | Leaking crankcase ventilation system

2. If all conditions test normal, test the engine for a mechanical condition. Refer to Symptoms - Engine Mechanical .

#### DTC P0172

1. Verify that none of the following conditions exist:

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

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

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

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

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

• | The air intake duct for being collapsed or restricted possibly by a restricted air filter

• | Leaking fuel pressure regulator--Inspect for evidence of fuel in the vacuum line. Refer to Fuel System Diagnosis .

• | Excessive fuel in the crankcase

• | The incorrect operation of the evaporative emissions control system

2. If all conditions test normal, test the engine for a mechanical condition. Refer to Symptoms - Engine Mechanical .

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Fuel Pump Replacement

• | Fuel Injection Fuel Rail Assembly Replacement