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Home / Codes / P0685
DTC code page

P0685: ECM/PCM Power Relay Control Circuit/Open

Quick answer: The PCM detected an open or missing control path for the engine-control power relay.

Drivers also search this fault as PCM power relay circuit open, ECM relay open circuit, engine control relay fault, PCM relay code.

Severity: high Family: powertrain Related paths: 15
Meaning

What P0685 usually means

P0685 usually means the engine computer is not happy with the relay circuit that is supposed to feed or control PCM power. In real diagnosis, this code often sits between a true relay failure and a more basic voltage-path problem such as a blown fuse, weak ignition-feed signal, corroded relay socket, or wiring fault that opens under heat or vibration.

Fast triage

Start here before chasing parts

  • Scan first: save freeze-frame and pending codes before clearing anything.
  • Confirm the complaint: compare the stored code with current drivability symptoms.
  • Use context: trims, live data, and related codes usually narrow the fault faster than guesswork.
  • Work simplest to hardest: leaks, connectors, maintenance items, and known patterns before expensive components.
Initial checks

What to check first

  • Verify whether the PCM is actually powering up consistently before blaming sensors that depend on it.
  • Check battery voltage, main fuses, and the relay socket for heat damage or looseness.
  • If the fault is intermittent, wiggle-test the relay and fuse-box area because this code often hides a connection problem rather than a dramatic component failure.
Driving risk

Can you keep driving?

Treat P0685 as a serious power-supply fault. If the engine stalls, cranks without starting, or loses PCM communication, the vehicle may become unreliable without warning.

High urgency: If symptoms are active, reduce driving and diagnose quickly before secondary damage builds.
Likely causes

Common causes behind this code

  • Failed ECM/PCM power relay or internally burned relay contacts
  • Open circuit between the ignition switch, relay control side, and PCM
  • Blown fuse or fuse-box connection problem feeding the relay
  • Corroded, spread, or heat-damaged relay socket terminals
  • Battery-voltage drop or poor ground creating a false relay-control story

Cause phrases often tied to this code: power relay, relay socket, ignition feed, blown fuse, battery voltage, corroded wiring.

Diagnostic order

Suggested workflow

  1. Confirm whether the engine computer communicates normally during the fault event.
  2. Check battery state and voltage drop during crank so a low-voltage event does not masquerade as a relay failure.
  3. Inspect the ECM/PCM power relay, fuse feeds, and relay socket terminals for heat, corrosion, or poor fit.
  4. Measure whether the control side and output side of the relay behave as expected when the key is turned on.
  5. Repair the verified voltage-path fault first, then retest for stable starts and repeat code return.
Avoid guesswork

Common mistakes

  • Replacing the PCM before proving the relay circuit can actually deliver stable power.
  • Ignoring fuse-box heat damage because the relay itself clicks.
  • Chasing crank or cam sensor codes first when the control module may not be staying powered.
Repair path

Practical fix guidance

  • Fix the confirmed open circuit, poor relay connection, fuse feed, or failed relay in the PCM power path.
  • After the repair, verify the PCM wakes up cleanly on repeated hot and cold starts.
  • If related low-voltage codes remain, continue into charging and battery-condition checks instead of calling the repair finished too early.
Vehicle context

Affected brands in this MVP

Brand hubs help broaden internal linking now and can evolve into make-specific diagnostic notes later.

Aliases and common searches

English phrases tied to P0685

Useful when the driver knows the wording but not the exact DTC yet.

  • PCM power relay circuit open
  • ECM relay open circuit
  • engine control relay fault
  • PCM relay code
Related search intent

Queries this page can answer naturally

  • P0685 code meaning
  • what does P0685 mean
  • PCM power relay open
  • ECM power relay control circuit open
FAQ

Quick questions about P0685

Can a bad relay really cause a crank-no-start?

Yes. If the PCM power relay does not feed the control module correctly, the engine may crank normally but never get consistent control power.

Does P0685 always mean the relay itself is bad?

No. Relay sockets, fuse feeds, ignition-switch input, and voltage-drop problems are common enough to test first.

Why can P0685 come and go with bumps or heat?

Because marginal relay terminals and fuse-box connections often fail intermittently under vibration or temperature change.