P0685🔧 Engine computer power supply (PCM power relay)Stop and fix promptly

P0685: ECM/PCM Power Relay Control Circuit Open (Meaning, Causes, Fixes)

Reviewed 2026-10-25 · Based on 2 cited source(s)

What P0685 means

Many OBD-II vehicles use a relay to feed battery voltage to the powertrain computer (PCM), though OBD-Codes.com notes a few use only a fused circuit. When the ignition is switched on, a small coil inside the relay is energized, its contacts close, and battery voltage flows out to the PCM. OBD-Codes.com says P0685 is stored when a low or no-voltage condition is detected in that supply circuit. RepairPal puts it more broadly: the PCM sets P0685 if it detects an incorrect voltage signal from the power relay. It is a generic code for 1996 and newer vehicles, but OBD-Codes.com notes the cause can differ a little between brands. The sources differ in emphasis. OBD-Codes.com lists fuses, fusible links, the ignition switch and battery cable ends along with the relay and wiring, while RepairPal lists only PCM failure, power relay failure and wiring. RepairPal also lists a hard or no start in cold weather, which OBD-Codes.com does not mention. Related codes P0687 through P068B cover other faults in the same relay's control and sense circuits.

Severity: can I keep driving?

Stop and fix promptly

Get it fixed right away. RepairPal rates P0685 Urgent, meaning immediate action is needed, and lists a no-start or a hard start in cold weather. OBD-Codes.com says a no-start will most likely come with this code, so ignoring it usually isn't an option. If the engine still starts and runs, OBD-Codes.com points to the engine computer itself or a programming error, so have it diagnosed before you rely on the car.

Common symptoms

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

  1. Bad PCM power relay (OBD-Codes.com, RepairPal)
  2. Blown fuse or fusible link (OBD-Codes.com)
  3. Corroded or damaged wiring or connectors, especially near the relay (OBD-Codes.com); wiring issue (RepairPal)
  4. Faulty ignition switch, or a loose terminal at the ignition switch (OBD-Codes.com)
  5. Loose or corroded battery cable ends (OBD-Codes.com)
  6. Faulty engine computer (PCM) or a programming error (OBD-Codes.com, RepairPal)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

  1. Check for technical service bulletins (TSBs) for your vehicle first; OBD-Codes.com notes your problem may be a known issue with a known fix.
  2. Look over the relay box, wiring and connectors. OBD-Codes.com says to watch for relays that have slipped out of their sockets or have corroded blades, especially when the relay box sits near the battery or coolant tank. Check that the battery cable ends are tight and clean.
  3. Check the fuses and fusible links that feed the PCM relay. OBD-Codes.com suggests testing fuses with the circuit loaded, and says swapping in an identical relay (matching part numbers) is a handy test. Put relays back where they came from and replace a bad one with a new one.
  4. With a multimeter and a wiring diagram, OBD-Codes.com describes checking the relay for battery voltage, for ignition-on voltage, for a good ground and then for output voltage to the PCM. No output with good inputs points to the relay; output at the relay but none at the PCM points to wiring. Unplug the computers before measuring resistance.

Typical fixes

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

  • The car won't start and you can't find a bad relay, fuse or battery connection
  • You don't have a multimeter or a wiring diagram for the PCM power circuit
  • A fuse keeps blowing after you replace it
  • Tests point to the engine computer, which may need programming to your vehicle

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P0685?

Usually you can't, because OBD-Codes.com says a no-start most likely comes with it. RepairPal rates it Urgent. If the car does start and run, get it diagnosed soon; OBD-Codes.com says that points to the engine computer or its programming.

Is P0685 just a bad relay?

Often, but not always. Both sources list the power relay and wiring, and OBD-Codes.com adds blown fuses or fusible links, the ignition switch and loose or corroded battery cables. Swapping in an identical relay is a quick test before buying parts.

Can a bad battery connection cause P0685?

OBD-Codes.com lists loose or corroded battery cable ends as a possible cause and says to check them early. RepairPal doesn't mention it.

How much does it cost to fix P0685?

Neither source gives a repair cost for P0685. It can be a relay, a fuse, a cable end or wiring, so diagnose it before buying parts.

Sources

  1. OBD-Codes.com: P0685 ECM/PCM Power Relay Control Circuit Open
  2. RepairPal: P0685 ECM/PCM Power Relay Control Circuit/Open (rated Urgent)

Last reviewed 2026-10-25. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.