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DTC codes / P06xx · Computer and output circuits / P0685

P0685ECM/PCM Power Relay Control Circuit Open

P0685 means the engine control module (ECM) or powertrain control module (PCM) has detected an open circuit in the control wire for its own power relay, so the relay cannot be commanded to supply power to the module. The computer is not reporting a failed relay directly; it is reporting that the electrical path it uses to switch that relay on is broken or not responding as expected.

SystemPowertrain
Code typeGeneric (all makes)
SeverityHigh
Fix difficultyProfessional
AreaComputer and output circuits

Symptoms you may notice

  • The check engine light or service engine soon light is illuminated.
  • The engine may crank but not start, or may start and then stall immediately.
  • Intermittent no-start conditions, sometimes with the engine restarting after the key is cycled.
  • The engine may run poorly or shut off while driving if the relay control circuit opens intermittently.
  • Other control modules may show communication codes because the ECM/PCM loses power.
  • Often no symptoms besides the warning light when the open is in a non-critical section of the control circuit.

Common causes, most likely first

  1. A failed ECM/PCM power relay, where the coil or internal contacts have opened.
  2. An open or corroded wire in the relay control circuit between the ECM/PCM and the relay coil.
  3. A poor connection at the relay socket, ECM/PCM connector, or an inline connector in the control circuit.
  4. A blown fuse that supplies power to the relay coil or to the ECM/PCM power circuit.
  5. A faulty ignition switch or power mode signal that prevents the relay from being commanded on.
  6. A damaged ECM/PCM driver circuit that can no longer ground or power the relay coil.
  7. Rodent damage or chafed wiring that has severed the control circuit conductor.

How to diagnose and fix it

  1. Verify the battery is fully charged and check all fuses related to the ECM/PCM power relay and module power supply.
  2. Inspect the relay socket and ECM/PCM connectors for corrosion, loose terminals, or pushed-out pins.
  3. Swap the ECM/PCM power relay with an identical known-good relay and clear the code to see if it returns.
  4. Use a wiring diagram to check for continuity and voltage on the relay control circuit between the ECM/PCM and the relay coil.
  5. Test the relay control circuit for a short to ground or short to voltage that could damage the ECM/PCM driver.
  6. If the control circuit is intact, test the ECM/PCM driver output with a logic probe or oscilloscope while commanding the relay on.
  7. Repair any open or damaged wiring, replace failed connectors, and clear codes to confirm the fault is resolved.

What the relay control circuit really means

  • P0685 is often set by a relay that has mechanically failed rather than by a wiring problem, so swapping the relay with a known-good unit is a fast and reliable first test.
  • A weak or failing battery can cause this code because the ECM/PCM may drop out during cranking and lose the ability to monitor its own power relay control circuit.
  • The code does not always mean the ECM/PCM is bad; a simple open in the control wire or a corroded terminal is far more common than a failed module.
  • Some vehicles use a power relay that is integrated into the underhood fuse box, so the entire fuse box assembly may need replacement if the relay is not serviceable separately.
  • Intermittent P0685 codes that appear only in cold or wet weather often point to a chafed wire or a connector with water intrusion rather than a failed component.

Can I keep driving with P0685?

Do not continue driving with an active P0685 because the ECM/PCM can lose power without warning, causing the engine to stall and leaving you without power steering or braking assist. If the code is intermittent and the engine runs normally, drive only to a repair shop and avoid high-speed or heavy-traffic situations.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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