Comprehensive diagnostic guide for OBD-II code P0654 Powertrain Control Module power relay control circuit

Important Notes

• do not include a vehicle-specific definition for P0654. However, Wikipedia's OBD-II sections explain that diagnostic trouble codes (DTCs) are generated by onboard computers to indicate faults and that many codes are grouped under Powertrain codes, i.e., PCM-related issues. This guide uses that framework to outline a practical, safety-focused diagnostic approach for a PCM power relay control circuit issue in a manner consistent with how P0xxx/Powertrain codes are treated in OBD-II systems. Where model-specific details would normally matter, this guide emphasizes general electrical power/relay logic as the typical root cause pattern for P0654.

What This Code Means

• In the realm of OBD-II powertrain codes, P0654 is associated with a fault in the PCM power relay control circuit (i.e., the circuit that the PCM uses to energize or control the main power relay that supplies power to the PCM and possibly other critical systems). The exact wiring and control logic can vary by vehicle, but the common theme is an abnormal condition in the circuit that the PCM uses to switch or control its own main power supply via a relay.
• The symptom pattern for this class of codes often involves power delivery issues to the PCM, which can manifest as no-code-triggered failures such as no-start, loss of communication with the PCM, intermittent operation, or a MIL with persistent or intermittent DTCs. Always verify against the vehicle's service information for exact pinouts and relay wiring.

Symptoms

• MIL illumination with P0654 stored or pending
• Engine may crank without starting or fail to crank if PCM is not getting power or is not able to control the main relay
• Intermittent no-start or stalls, especially after initial crank or during vehicle operation
• Loss of communication with other modules (no PCR/ECU data link) due to PCM power instability
• Parasitic battery drain if the relay or circuit causes the PCM to stay energized or to cycle inappropriately
  Note: Symptoms can vary by make/model; the service manual for the specific vehicle should be consulted for exact symptom trees.

Critical safety reminders

• P0654 relates to power/relay circuits that affect the PCM. Work carefully around the battery and wiring. Disconnect or disconnect appropriate fuses if performing invasive electrical tests.
• Always perform tests with the ignition off, then with the ignition on as needed, and while monitoring live data safely. Use proper PPE and ensure the vehicle is securely supported if you access under-hood wiring.

Diagnostic Approach

1) Confirm the DTC and collect context

• Use a capable scan tool to confirm P0654 is active or stored. Note freeze-frame data, any related codes (e.g., P0600 PCM communication, P0685 etc.), and recent freeze-frame values for voltages, temperatures, and RPM.
• Check for any related codes from the Powertrain category that could point to a shared power/ground issue or relay misbehavior.

2) Vehicle readiness, power, and ground checks

• Battery condition: measure resting voltage (should typically be ~12.6 V or higher for a healthy battery) and charging system output (14.0-14.8 V while running).
• Inspect battery terminals and grounds: clean, tight connections; inspect chassis ground strap integrity.
• Check major fuses and the main power feed fuse related to the PCM and ignition-fed circuits; verify continuity.

3) Visual and mechanical inspection of power-relay circuit

• Locate the main power relay or the relay pack that feeds the PCM. Identify the relay coil control wire(s) and the power/ground legs for the relay contacts.
• Inspect relay socket for corrosion, bent pins, or poor connections. Wiggle test connectors (with ignition OFF) to feel for looseness or arcing risk.
• Inspect wiring harnesses to the PCM and relay: look for signs of chafing, heat damage, opened insulation, or water intrusion. Look for melted insulation or signs of short-to-power or short-to-ground hazards.
• Check for aftermarket harnesses or adapters that could affect relay control circuits.

4) Relay coil and control path testing (coil drive by PCM)

• With ignition ON (engine OFF), measure the relay coil voltage at the relay coil terminals:
  • One side should read battery voltage (or a constant feed depending on design).
  • The other side should toggle to ground or to a switching control signal from the PCM (as per vehicle service data) when commanded.
• If the coil does not energize when the PCM is supposed to energize it, suspect:
  • Faulty relay (internal coil open or high resistance)
  • Faulty coil drive signal from PCM
  • Open circuit in the coil power feed or a blown fuse
• Relay coil resistance is typically in the low ohms range (per the OEM spec). Compare measured resistance with the service data.

5) Main power feed to PCM

• Verify the continuous VBAT feed to the PCM (constant power) and the ignition-switched feed (VIGN) that powers the PCM when the ignition is on.
• With a multimeter, check for voltage at the PCM power pins in both key-on and key-off states according to the vehicle's service data.
• If VBAT is missing or VIGN is absent or unstable, this supports a power feed issue rather than a PCM internal fault.

6) Ground integrity checks

• Verify PCM ground connections (socket pins and chassis grounds) for continuity to a solid ground. A poor ground can mimic PCM power issues.
• Check for corrosion or loose connections on ground circuits that could cause the PCM to reset or fail to energize the control circuit.

7) Wire harness and connectors

• Inspect for chafed insulation or damaged wires in the relay control circuit and main power feed. Use a no-load resistance or continuity test to confirm intact paths to the PCM and relay.
• Check for any short-to-ground or short-to-power conditions on the relay control lead and on the PCM power feeds.

8) Isolate the PCM or relay as the fault

• If the power/ground and relay control paths test good, and there are no harness faults, consider PCM internal fault or software issue as a possibility.
• In some cases, PCM replacement or reprogramming/updated calibration may be required, but only after thorough checks of the harness and relays to avoid repeating the failure.

9) Vehicle-specific checks and re-tests

• Consult the vehicle's service manual for exact pin locations, wire color codes, and expected voltages for the PCM power feed and relay coil circuits. The fault can be model-specific in how the PCM energizes the main power relay.
• After repairs, clear the codes, perform a drive cycle, and re-scan to confirm the DTC does not return. If DTC reappears, re-evaluate all power, relay, and ground paths and consider PCM reflash as a later step if appropriate.

Testing procedures (practical measurement targets)

• Battery voltage: 12.6 V or higher at rest; 13.8-14.8 V while cranking/idle with engine running.
• Relay coil resistance: check against OEM spec; typically a few ohms; high resistance or open coil indicates a bad relay.
• Relay contact voltage: when relay is energized, measure that the relay's main contact terminal provides a stable feed to the PCM (per service data).
• Continuity tests for VBAT and VIGN paths to the PCM power pins; no gaps or intermittent opens.
• Ground path tests: low resistance between PCM ground pins and vehicle chassis ground.

Causes and likelihood

• Faulty main power relay or its coil drive path (highest likelihood): roughly 40%
• Open or high-resistance wiring in the PCM power/relay circuit (including harness or connector fault): about 25%
• Battery/charging or poor ground condition causing the PCM to fail to energize or communicate: about 15%
• PCM internal fault (hardware or firmware issue) or need for software calibration/update: about 10%
• External modules or backfeeds causing abnormal relay behavior (less common): about 10%

What not to assume or do

• Do not replace the PCM as the first step without exhausting all power/relay circuit checks. A relay, fuse, or wiring fault is far more common than a PCM failure.
• Do not overlook the battery/charging system. A failing alternator or a dying battery can cause symptoms that mimic PCM power issues.
• Do not overlook grounds. A shared or poor ground path can cause PCM power/relay symptoms that appear to be a relay problem.

Documentation

• Record all voltage readings, relay coil resistances, and continuity results with timestamps.
• Take photos of wiring harnesses and connectors before disconnecting anything to ensure correct reassembly.
• Keep the freeze-frame data and any related codes for correlation if symptoms recur.

Safety and tooling notes

• Disconnect the battery when performing invasive electrical tests near live circuits if required by the procedure you're following.
• Use insulated tools, proper PPE, and take care with ignition systems and airbag/other safety-sensitive circuits if applicable.
• Ensure the vehicle is in a well-ventilated area and the parking brakes are engaged when performing under-hood inspections or battery work.

Vehicle-specific follow-up

• Because exact P0654 wiring, pinouts, and control logic can vary by manufacturer and model, always consult the vehicle's service manual for precise relay control circuit diagrams, pinouts, and expected voltage states.
• If a repair is completed based on this guide, perform a thorough road test and monitor for reoccurrence of the DTC, and re-scan after a few cycles to ensure the fault does not return.

What the sources say

• OBD-II overview and diagnostic trouble codes context: DTCs are produced by onboard systems to indicate faults; understanding DTCs is crucial for troubleshooting.
• Powertrain codes are a subset of OBD-II codes and reflect issues in the PCM/engine control domain; they form a major category for troubleshooting powertrain-related faults.
• General OBD-II emission/test context notes emphasize the role of PCM and related systems in monitoring and reporting faults.

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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