Comprehensive diagnostic guide for OBD-II P0685 ECM/PCM Power Relay Control Circuit - Open Circuit

What This Code Means

• What the code means: P0685 denotes an open circuit in the ECM/PCM/TCM power relay control circuit. In practical terms, the engine control module (ECM/PCM/TCM) is not receiving a valid controlled feed to its power relay circuit, or the ECM is not providing the control signal to the relay coil. The open circuit condition prevents the main power relay from energizing, which can starve the ECM and other vital circuits of power.
• Source reference: Definition aligned with the open-source description (ECM/PCM/TCM power relay control circuit open). This aligns with the intended meaning of P0685 in the OBD-II framework described by open-source repositories.
• Broader context (OBD-II): DTCs are generated by onboard monitoring systems in modern vehicles to indicate issues detected in powertrain and related emissions-related systems. Powertrain codes include various ECM/PCM-control circuit faults.

Symptoms

• No start or hard no-start condition despite a healthy battery (ECM not receiving proper power). Users may report "cranks but won't start" due to the ECM not powering up the fuel/ignition systems.
• Intermittent no-start or intermittent stalling with loss of ECM power control.
• Scan tool shows limited or no ECU data, or the ECU appears not to be powering up consistently.
• In some cases, vehicle starts then dies, or the engine cranks but the ECU does not command fuel/injection because the relay control circuit is open.
• Note: Symptoms may be influenced by electrical system health (battery voltage, grounding) and may appear alongside other powertrain codes if the main feed to the ECM is compromised.

Probable Causes

Because do not include specific NHTSA complaint statistics for P0685, the following probabilities are based on typical field experience and the nature of relay-control circuits. Treat these as practical starting points rather than exact distributions:

• Open or damaged relay control circuit wiring (fuse, fusible link, harness, connectors): ~30%
• Faulty ECM/PCM relay or relay coil (stuck open or failing to energize): ~25%
• Faulty or corroded power/ground wiring to ECM (or to the relay control side): ~20%
• Faulty ECM/PCM internal control circuit (ECM trying to drive the relay but internal fault or processor issue): ~15%
• Other causes (intermittent harness damage, improper aftermarket wiring, accessory loads causing voltage drop, battery/charger issues influencing relay operation): ~10%

Safety Considerations

• Always disconnect power and follow anti-spark procedures when inspecting or replacing relays, fuses, and wiring in the engine bay.
• If you must crank or start the engine for diagnostic purposes, ensure the vehicle is in a safe, secure location and foot on the brakes as required; avoid contact with moving parts.
• When testing, avoid shorting battery terminals or applying high-current tests to fragile ECM connectors or wiring harnesses.

Diagnostic Approach

1) Prepare and verify

• Confirm P0685 is present with a current, valid code read. Note any related codes (P0684, P0690, P0673 family codes, etc.) that might indicate related power relay/ECM issues.
• Review freeze-frame data (engine load, voltage, temperature) if available to assess conditions under which the fault occurred.
• Verify battery condition and charging system. A weak battery can cause relay operation issues and intermittent PCM power. Ensure at least ~12.6 V with engine off and ~13.8-14.8 V with the engine running.

2) Visual inspection

• Inspect ECM/PCM power relay and its fuse(s) for signs of being blown, burnt, or corroded connectors.
• Inspect the relay control wiring from the ECM/PCM to the relay coil for damaged insulation, exposed conductors, or loose/bent pins at the connector.
• Check ground connections to the ECM/PCM and to the relay control circuit; ensure solid grounds and clean connectors.
• Look for aftermarket wiring or modifications that could affect the relay circuit.

3) Verify power to the ECM/PCM and relay operation

• With key on (not necessarily engine running), check for voltage at the ECM power input pin(s). There should be stable battery voltage or a controlled feed as designed by the vehicle (per wiring diagram).
• Check the control side of the relay coil: when the ECM commands the relay (or when the engine is cranking if the design uses cranking circuitry), the relay coil should energize and the relay should click. You can listen for the relay and feel for a click, or measure coil voltage with a DVOM/oscilloscope.
• If the relay coil energizes normally but the relay does not switch power to the ECM, suspect wiring between the relay and ECM power input, or an ECM fault on the power input path.

4) Probe the relay coil circuit (ECM control circuit)

• Identify the ECM pin that drives the relay coil. Measure the control voltage when the ignition is on and during crank (if the circuit is designed to energize during cranking). If the ECM is not providing the expected control signal, suspect ECM fault or a grounding/communication issue affecting the ECM's ability to drive the relay.
• If the relay coil is energizing but the ECM still reports P0685, re-check the main power feed and essential grounds to the ECM; a poor main supply or ground can cause the ECM to behave incorrectly even if the coil side looks OK.

5) Inspect wiring between relay, ECM, and power supply

• Look for damaged, pinched, or corroded wires, especially in the harness around the relay socket and ECM connectors.
• Check for damaged insulation, heat damage, or oil/chemical exposure that could cause intermittent open circuits.
• Wiggle test connectors (with ignition on and engine cranking as appropriate) to identify intermittent opens. If the fault reappears or disappears with movement, wiring or connector faults are likely.

6) If the relay and wiring test normal, consider PCM-level causes

• If the relay coil energizes properly and the power feed to the ECM is present and stable, yet P0685 persists, the ECM/PCM's internal control circuitry may be at fault. This is less common but possible.
• In this scenario, you may consider testing with a known-good ECM/PCM (where feasible) or replacing the PCM if permitted by the vehicle's security and configuration, after ruling out wiring/fusing issues. Note that PCM replacement involves reprogramming and could trigger security/immobilizer considerations; consult service documentation.

7) Verification after repair

• Clear the DTCs and perform a test drive under varying conditions (idle, light load, higher load, cold/hot starts) to verify the fault does not reappear.
• Monitor for any reappearance of P0685 or related power-relay/control codes.
• Confirm that the ECM powers properly when key-on and remains powered during engine operation if that is how the circuit is designed.

Recommended tests and practical checks (quick-reference)

• Visualize: Fuse integrity and relay integrity; connector cleanliness; corrosion-free terminals.
• Electrical tests: Battery voltage, relay coil resistance, coil activation voltage, and continuity of the control circuit from ECM to relay.
• Functional tests: Relay energization test (bench test or controlled ignition-on test) and confirming power to the ECM when the relay is energized.
• Diagnostic data: If accessible, capture ECM relay control signal with scope to verify proper switching behavior; check for erratic control pulses or missing signals.

Repair Options

• Replace blown fuse or damaged fusible link feeding the ECM power supply.
• Replace faulty ECM/PCM power relay or correct relay coil circuit (if coil is weak or intermittently fails to energize).
• Repair or replace damaged wiring, harness, or connectors between the relay and the ECM, including repairing grounds and battery negative connections as needed.
• Repair/replace ECM/PCM if an internal fault in the controller's relay control circuit is confirmed after excluding wiring and external faults.
• Re-check all work after repair and clear codes; perform a road test to confirm reliability.

What to document (for service records)

• Code(s) observed (P0685) and any related codes.
• Vehicle make/model/year, mileage, and exact fuse/relay part numbers replaced.
• Observed symptoms, battery/charging condition, and environmental conditions at failure.
• Diagnostic steps performed, data readouts (freeze-frame, relay voltage, ignition on/off), and final test results.
• Any PCM programming steps, immobilizer considerations, or requiring dealer-level confirmation for PCM replacement.

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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