Meaning of GMC P128F Code
The fuel rail pressure sensor transmits fuel pressure and temperature information as serial data.
The sensor uses an internal microprocessor to generate four separate outputs from a three-wire sensor. The Engine Control Module (ECM) powers this sensor via:
- A 5V reference circuit,
- A low reference circuit,
- An asynchronous signal/serial data circuit.
The asynchronous signal means communication occurs only from the sensor to the ECM. The ECM then decodes this data into separate voltages, which are displayed on a scan tool as input voltages for:
- The fuel temperature sensor,
- The fuel rail pressure sensor,
- The fuel rail pressure sensor 2.
Possible Symptoms
- Check engine light on (or “Service Engine Soon” warning light).
Possible Causes
- Faulty fuel rail pressure sensor
- Open or short circuit in fuel rail pressure sensor harness
- Poor electrical connection in fuel rail pressure sensor circuit
Diagnostic and Repair Procedures
1. Visual Inspection
- Check the condition of the wiring harness and related connectors to the fuel rail pressure sensor.
- Look for damaged wires, broken, bent, pushed out, or corroded connector pins.
- Ensure electrical connections are securely fastened and free of corrosion.
2. Fuel Rail Pressure Sensor Check
- Use a digital multimeter to measure the sensor’s output voltage.
- Compare the obtained values with manufacturer specifications.
- Replace the sensor if the values are out of tolerance.
3. Wiring Harness Check
- Test the wire continuity between the ECM and the fuel rail pressure sensor.
- Check for short circuits or open circuits using a multimeter in ohmmeter mode.
- Repair or replace any damaged wiring.
4. Engine Control Module (ECM) Check
- If all previous checks are correct, it may be necessary to test the ECM module.
- Consult a qualified technician or specialized repair center for advanced diagnostics.
Conclusion
The GMC P128F code indicates a problem related to the fuel rail pressure sensor or its electrical circuit. A thorough inspection of connections, harness, and sensor voltages will help identify the cause of the issue and resolve it effectively.