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 Bank 1
DTC P0172: Fuel Trim System Rich Bank 1
DTC P0174: Fuel Trim System Lean Bank 2
DTC P0175: Fuel Trim System Rich Bank 2

### 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, P0030, P0031, P0032, P0036, P0037, P0038, P0050, P0051, P0052, P0056, P0057, P0058, P0102, P0103, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0131, P0132, P0133, P0134, P0137, P0138, P0139, P0140, P0151, P0152, P0153, P0154, P0157 P0158, P0159, P0160, P0221, P0222, P0223, P0230, P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0273, P0274, P0276, P0277, P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0335, P0336, P0337, P0340, P0443, P0458, P0459, and P2119 are not set.

• | The fuel system is in Closed Loop.

• | The engine speed is greater than 512 RPM.

• | The mass air flow (MAF) is greater than 49 mg/stk.

• | The engine coolant temperature (ECT) is greater than 45°C (113°F).

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

• | The intake air temperature (IAT) sensor is greater than -39°C (-39°F).

• | The EVAP canister has been purging for greater than 240 seconds, then it is disabled for 60 seconds.

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

### Conditions for Setting the DTC

#### P0171 or P0174

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

#### P0172 or P0175

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

### Action Taken When the DTC Sets

DTCs P0171, P0172, P0174, P0175 are Type B DTCs.

### Conditions for Clearing the MIL/DTC

DTCs P0171, P0172, P0174, P0175 are Type B DTCs.

### Diagnostic Aids

• | 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.

• | 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

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. DTCs P0171 or P0172 should not set.
2. Ignition ON, observe the scan tool 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 P0107 or P0108.

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

⇒ | If greater than the specified range, refer to DTC P0121, P0122, P0123, P0221, P0222, or P0223.

4. Observe the scan tool HO2S sensor voltage parameter. The reading should fluctuate within 200-800 mV.

⇒ | If not within the specified range, refer to DTC P0131 or P0151 and DTC P0137 or P0157.

5. Engine at normal operating temperature, observe the scan tool Long Term FT parameter. The parameter should be between -5 to +5 percent.
6. 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.

### Circuit/System Testing

1. Verify none of the following conditions exist:

#### P0171 or P0174

• | 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

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

• | 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.

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

#### P0172 or P0175

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

• | 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.

• | 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.

• | Restricted exhaust components--Refer to Symptoms - Engine Exhaust.

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 Injection Fuel Rail Assembly Replacement

• | Air Cleaner Element Replacement