The error code P1238 indicates a Fuel Pump Secondary Circuit Malfunction. This diagnostic trouble code (DTC) is closely related to P1237, sharing the same root causes but with a key difference in some vehicle applications: P1238 may not always trigger the Malfunction Indicator Lamp (MIL) on your dashboard. However, user experiences can vary, and illumination of the MIL with a P1238 code is possible.
This code signals that the Fuel Pump Driver Module (FPDM) has detected a fault within the fuel pump’s secondary circuit. The FPDM’s role is to manage the fuel pump’s speed by adjusting the ground on the Fuel Pump Return (RTN) circuit. When it detects an issue, it communicates this to the Powertrain Control Module (PCM) via the Fuel Pump Monitor (FPM) circuit. Upon receiving this signal, the PCM sets the P1238 DTC.
Several potential issues can lead to a P1238 code:
- Open or Shorted Fuel Pump Power (FP PWR) Circuit: This is a common culprit. An open circuit means a break in the wiring, preventing power from reaching the fuel pump. A short circuit implies unintended electrical contact, potentially diverting power or causing damage. Issues can range from damaged wires, corroded connectors, or faulty wiring harnesses.
- Open Fuel Pump Return (FP RTN) Circuit to FPDM: The fuel pump return circuit is crucial for completing the electrical loop. An open in this circuit disrupts the FPDM’s ability to control the fuel pump effectively.
- Open or Shorted Circuit within the Fuel Pump: Problems within the fuel pump itself, such as internal wiring faults or shorts, can trigger the P1238 code.
- Locked Fuel Pump Rotor: A mechanical issue where the fuel pump rotor is seized or unable to rotate freely. This can place excessive strain on the electrical circuit and be flagged as a circuit malfunction.
- Damaged FPDM (Fuel Pump Driver Module): If the FPDM itself is faulty, it may incorrectly detect or report circuit malfunctions.
Special Note on LS6/LS8 Engines:
The original text mentions LS6/LS8 engines, noting that in these General Motors (GM) applications, the FPDM functionalities are integrated into the Rear Electronics Module (REM). Furthermore, these systems might not utilize a dedicated FPM circuit. Diagnostic information in these cases is often communicated via Serial Communication Protocol (SCP). If you are working on a vehicle with an LS6 or LS8 engine, be aware of these system differences.
Understanding the FPDM Communication:
When the FPDM detects a secondary circuit fault, it transmits a 75% duty cycle signal to the PCM through the FPM circuit. This signal pattern (750 milliseconds ON, 250 milliseconds OFF) alerts the PCM to the fault. Once the fault condition is resolved, the FPDM reverts to sending a 50% duty cycle signal (500 milliseconds ON, 500 milliseconds OFF), indicating an “all OK” status. Even if the fault is no longer active, the PCM will typically store the P1238 code in continuous memory.
Initial Troubleshooting Steps:
A practical first step in diagnosing a P1238 code, as suggested in the original text, is to check for a blown fuse related to the fuel pump circuit. A blown fuse is a common cause of an open circuit condition. Inspect your vehicle’s fuse box diagram to locate the fuel pump fuse and check its condition.
Further investigation may involve:
- Inspecting wiring and connectors for damage, corrosion, or loose connections in the fuel pump power and return circuits.
- Testing circuit continuity and voltage using a multimeter to pinpoint open or short circuits.
- Evaluating fuel pump functionality and considering potential mechanical issues like a locked rotor.
- Diagnosing the FPDM itself to rule out internal module failure.
Resolving a P1238 code requires a systematic approach to identify and address the underlying cause within the fuel pump secondary circuit.
