Comprehensive Diagnostic Guide for OBD-II Code P1238

Fuel Pump Secondary Circuit Malfunction

What This Code Means

• Meaning entry titled .
• Where it sits in the system: P1238 is a powertrain/DTC, i.e., a diagnostic trouble code reported by the vehicle's PCM related to engine/fuel system operation (powertrain control). This aligns with the general structure of OBD-II DTCs described in the Wikipedia OBD-II pages.
• Basic concept: The PCM monitors fuel delivery circuits, including secondary pump circuits, and sets P1238 when it detects a fault condition in that circuit. This is consistent with how OBD-II powertrain codes are described as monitoring and fault-detection mechanisms for engine/emission-related systems. Sources: Wikipedia entries on Diagnostic Trouble Codes and Powertrain Codes.

Reality check: why this code appears

• The fault typically points to issues in the power delivery path or control path to the secondary fuel pump (or its control electronics). It can be triggered by:
  • Pump relay failure or pump itself failing to draw current
  • Faulty wiring, connectors, or grounds in the secondary pump circuit
  • Fuel pump power supply fuses or fuselinks
  • PCM/ECU drive/sensor signal anomalies
  • Intermittent faults that only show up under load or during cranking
• These general failure modes align with the diagnostic framework for powertrain codes described by the OBD-II references and the typical symptoms observed with fuel-delivery circuit faults.

Diagnostic Approach

Tools you'll likely need

• OBD-II scanner capable of reading freeze-frame data and live data (fuel pressure, fuel rail pressure, pump current draw, voltage, duty cycle, PCM duty/controls)
• Fuel pressure gauge or appropriate diagnostic tool for your vehicle
• Multimeter and test leads
• Access to wiring diagrams for the vehicle's fuel pump circuit
• Safety gear (gloves, eye protection; follow shop safety procedures for working around fuel systems)

Stage 1 - Verify and scope

• Confirm P1238 is present and note any related DTCs (fuel pressure, circuit faults, relay faults, ECM/PCM codes). The PCM stores DTCs for powertrain issues; verify there's no conflicting fault code that could mask the primary issue. Sources: Wikipedia - Diagnostic Trouble Codes; Wikipedia - Powertrain Codes.
• Review freeze-frame data (engine RPM, vehicle speed, engine load, fuel pressure, current to the pump, voltage to the circuit) to understand the fault condition at the time the code was set.
• If the MIL is on, check for OEM service bulletins or reprogram/flash updates that address fuel-pump-circuit behavior (not specified , but a standard step in many diagnostic workflows).

Stage 2 - Visual and basic electrical checks

• Inspect the secondary fuel pump circuit wiring harness, connectors, and grounds for corrosion, damage, pulled wires, or loose connections.
• Check all fuses and relays related to the fuel pump circuit, including the secondary pump circuit control relay if the vehicle uses one.
• Inspect the battery and charging system voltage; low voltage can cause pump under-volt or erratic operation.
  Note: These are standard electrical-system diagnostic steps consistent with exploring circuits described in the general OBD-II framework.

Stage 3 - Functional tests of the fuel delivery path

• KOEO (key-on, engine-off) test: verify whether the pump (or pumps) prime as ignition is turned on. Listen for a pump prime after turning the key to the ON position (the exact priming behavior varies by vehicle; many systems prime for 2-3 seconds). If there is no prime or a weak prime, suspect pump circuit power or the pump itself.
• Fuel pressure test:
  • With the engine OFF, verify baseline fuel pressure is within the manufacturer's specification. Then, during crank and/or run, observe pressure behavior. A significant drop or inability to reach spec pressure supports a pump, circuit, or restriction issue.
  • Compare observed values with the vehicle's spec and note any rapid pressure drop as load increases.
• Pump current/voltage check:
  • Monitor the pump power supply voltage and current draw during KOEO and cranking. A pump that does not draw expected current or has a voltage drop under load points to wiring, connector, relay, or pump motor concerns.
• Check for any PCM commands or driver faults to the pump relay or control signal; verify that the PCM is providing the proper duty cycle if the pump is controlled by the ECU.

Stage 4 - Narrowing down causes and related checks

• Faulty secondary pump or failed motor
  • Symptoms align with a motor that cannot sustain operation, especially under load or during cranking when the pump should be delivering fuel.
  • Consider testing/bench-testing the pump or substituting with a known-good unit if feasible.
• Wiring harness/connector faults in the secondary circuit
  • Corrosion, damaged insulation, or loose pins can cause intermittent failures that match P1238's behavior.
• Relay or fuse issues
  • A weak or failing relay or a blown fuse can produce symptoms similar to a pump fault.
• PCM/ECU drive fault or control signal issues
  • If the PCM fails to command the pump appropriately, or if a sensor input misbehaves and the ECU disables the pump, P1238 can be triggered.
• Secondary circuit vs primary circuit confusion
  • Some vehicles have a primary pump and a secondary circuit or a separate feed for high-demand conditions. Ensure you're testing the intended circuit for the specific make/model.

Stage 5 - Confirmatory tests and correlating data

• Recheck fuel pressure while performing a controlled load test (e.g., simulate acceleration or crank conditions) to see whether the pressure or pump drive changes as expected.
• Check for any related DTCs that could point to a fuel-pressure fault, wiring fault, or relay fault (e.g., P0087 fuel rail/system pressure too high/low, P0230 fuel pump secondary circuit faults, etc.). The presence of related codes can help pinpoint the root cause.
• Confirm that the issue is not caused by contaminated fuel, clogged fuel filters, or other fuel-path restrictions that could masquerade as a secondary circuit fault. Such conditions may influence fuel pressure test results.

Stage 6 - Repairs and remedies (prioritization)
Prioritize based on the most probable root causes identified during testing:

1) Fuel pump and/or pump relay/channels

• Replace the fuel pump assembly if the pump is failing or shows abnormal current draw/voltage behavior that cannot be resolved by wiring/relay repair.

2) Wiring harness, connectors, and grounds

• Repair or replace damaged wiring, repair grounds, and reseat connectors. Cleaning corroded terminals and ensuring secure connections can restore proper circuit function.

3) Fuses/relays

• Replace faulty fuses or relays with OEM-spec parts and re-check operation.

4) PCM/ECU control signal issues

• If tests indicate the PCM is not issuing the correct drive signal to the pump (or is misreading inputs), investigate PCM/software updates, or potential immobilizer/signal integrity issues. This may involve OEM service tooling or software reflash per vehicle-specific guidance.

5) Additional considerations

• If there is contamination in fuel (water/ingress) or a clogged fuel filter, address fuel quality and flow restrictions as needed. This may indirectly affect pump performance and circuit behavior.

Stage 7 - Post-repair verification

• Clear DTCs and perform a drive cycle that replicates the conditions under which P1238 previously appeared (e.g., cold start, idle, and under load).
• Confirm that fuel pressure now remains within spec during KOEO and during cranking and running conditions.
• Ensure no related DTCs reappear after a representative number of drive cycles.

Safety Considerations

• Always relieve fuel system pressure and follow proper procedure before disconnecting fuel lines or servicing the pump.
• Work in a well-ventilated area away from ignition sources.
• Use appropriate PPE and follow shop safety protocols when testing electrical circuits around the fuel system.
• Disconnect the battery as needed before servicing electrical components and avoid short circuits.

Probable-cause probability estimates

• Fuel pump itself or pump relay (primary suspect for P1238): 40%

• Wiring harnesses, connectors, and grounds in the secondary circuit: 25%

• Fuses/relays and intermittent power supply issues to the pump: 15%

• PCM/ECU drive signal or control issues: 10%

• Other mechanical factors (fuel quality, clogged paths) contributing to symptoms: 10%

• Diagnostic Trouble Codes and general OBD-II framework: Diagnostic Trouble Codes and Powertrain Codes describe how DTCs are used to monitor engine and emission-control systems and how the codes fit into the powertrain category. This supports the general approach of diagnosing P1238 as a powertrain/fuel-delivery issue.

• Code interpretation mapping: The Open Source entry translating to provides a direct mapping for P1238 as a secondary fuel pump circuit fault. Use this as the central meaning for the code in a diagnostic guide. - Bomba de combustível secundário - circuito defeituoso)

• Emissions/testing context: The OBD-II references to emissions testing emphasize that DTCs like P1238 have implications for emissions-related function and diagnostics, reinforcing the importance of fuel-delivery circuit integrity in engine performance and emissions compliance.

Notes on conflicts and data limitations

• There is no conflicting information between regarding the general concept of P1238 as a fuel-pump-secondary-circuit fault. The open-source code definition aligns with the idea of a circuit fault in the fuel-delivery path, and give the broader context on how DTCs operate within OBD-II and the powertrain domain.
• The sources do not provide vehicle-specific definitions, OEM fault trees, or NHTSA complaint data for P1238. The probability estimates above are and generic diagnostic reasoning in the absence of vehicle-specific data, as requested.

Notes

• Always correlate P1238 with other DTCs and data. A single DTC can be the symptom of a broader electrical or control-path problem.
• If repeated diagnostics fail to locate a fault, consider PCM software/firmware updates, OEM service bulletins, or potential immobilizer/anti-theft impacts that might indirectly influence fuel-supply controllability.
• Some vehicles require special procedures to access the fuel-pump circuit (e.g., removing rear-seat access panels, paneling, or trunk/fuel-tiller access). Follow OEM service documentation for vehicle-specific steps.

In summary

• P1238 indicates a fault in the fuel pump secondary circuit. Use a systematic electrical and fuel-delivery diagnostic approach: verify the code, inspect wiring and connections, test pump power, pressure, and current flow, correlate with related DTCs, and verify repair with a drive cycle. The diagnostic framework is consistent with the general OBD-II and powertrain code concepts described .

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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