Fuel Pump Relay Diagnosis

The fuel pump relay is located in the rear relay center next to the rear fuse block under the rear seat cushion. Refer to Driveability and Emissions for the locator view. For diagnosis of the fuel pump relay circuit, refer to DTC P0231 Fuel Pump Feedback Circuit Low Voltage.
Testing and Inspection

If there is a fuel delivery problem, refer to Fuel System Pressure Test. This test diagnoses the fuel injectors, the fuel pressure regulator, and the fuel pump. If a malfunction occurs in the fuel metering system, a rich HO2S signal or lean HO2S signal is a result. This condition is indicated by the HO2S voltage, causing the PCM to change the fuel calculation (fuel injector pulse width) based on the HO2S reading. Changes made to the fuel calculation will be indicated by a change in the long term fuel trim values which can be monitored with a scan tool. Ideal long term fuel trim values are around 0%; for a lean HO2S signal, the PCM will add fuel, resulting in a fuel trim value above 0% and for a rich HO2S signal the PCM will take away fuel, resulting in a fuel trim value below 0%. Some variations in fuel trim values are normal due to power train design. If the fuel trim values are excessively greater or lower then 0%, refer to DTC P0171 Fuel Trim System Lean Bank 1, DTC P0172 Fuel Trim System Rich Bank1, DTC P0l74 Fuel Trim System Lean Bank2, or DTC P0l76 Fuel Trim System Rich Bank 2.
Testing and Inspection
Always start with the Powertrain OBD System Check.
Testing and Inspection

If a malfunction occurs in the fuel metering system, it usually results in either a rich or lean exhaust condition. This condition is sensed by the HO2S, and causes the PCM to change the fuel calculation (injector pulse width). The change made to the fuel calculation is indicated by a change in the short term fuel trim and long term fuel trim values which can be monitored by a scan tool. A momentary change to the fuel calculation is indicated by the short term fuel trim value, while a prolonged change is indicated by the long term fuel trim value. Average fuel trim values are around 0%, but vary slightly from engine to engine.

IMPORTANT: When using a scan tool to observe fuel trim values, remember that if the system is in control, no action is required unless a driveability symptom is present. Listed below are examples of lean and rich conditions with the system in control and out of control.

^ A momentary lean condition (system is in control) will appear on the scan tool as:

- Short term fuel trim value above 0% (adding fuel).

- Long term fuel trim value around 0%.

^ A prolonged lean condition (system is in control) will appear on the scan tool as:

- Short term fuel trim value around 0%.

- Long term fuel trim value above 0% (added fuel).

^ A prolonged lean condition (system is out of control) will appear on the scan tool as:

- Short term fuel trim value well above 0% (adding fuel).

- Long term fuel trim value well above 0% (added fuel).

If both fuel trim values are fixed well above 0%, refer items which can cause a lean system.

^ A momentary rich condition (system is in control) will appear on the scan tool as:

- Short term fuel trim value less than 0% (reducing fuel).

- Long term fuel trim value around 0%.

^ A prolonged rich condition (system is in control) will appear on the scan tool as:

- Short term fuel trim value around 0%.

- Long term fuel trim value less than 0% (reduced fuel).

^ A prolonged rich condition (system is out of control) will appear on the scan tool as:

- Short term fuel trim value much less 0% (reducing fuel).

- Long term fuel trim value much less 0% (reduced fuel).

IMPORTANT: If both fuel trim values are fixed well below 0%, refer to items which can cause the system to run rich. If a driveability symptom exists, refer to the particular symptom in Symptoms for additional items to check.
Testing and Inspection