P0685 to P0687: ECM power relay control circuit

Switching off the ignition does not switch off the engine control module. It keeps running for a short period to finish housekeeping — writing adaptations to memory, running the cooling fan if needed, closing the throttle to its rest position.

A main relay, sometimes called the ECM power relay or system relay, controls that. And it is the module itself that holds the relay closed until it is ready to sleep.

Code	Meaning
P0685	ECM power relay control circuit / open
P0686	Circuit low
P0687	Circuit high

The two failure directions

The relay does not close

The module never gets power, or loses it too early. Symptoms:

• No start, sometimes intermittent.
• Stalling while driving, with an immediate restart that works: engine stalls while driving causes.
• Adaptations lost repeatedly, so the engine idles poorly after every start.
• Keep-alive memory codes: P0600 to P0605 control module memory.

The relay does not open

The module never sleeps. Symptoms:

• A battery flat after a few days parked. This is the classic presentation.
• The cooling fan running long after switch-off, or forever.
• A warm relay when the car has been parked for hours.

That second failure is the more common and the more expensive, because repeated deep discharge ruins a battery: parasitic battery drain diagnostics.

The overnight test

The most informative measurement, and it needs only patience.

1. Lock the car and leave it for at least an hour so all modules sleep.
2. Measure current draw at the battery, or watch resting voltage over several days.
3. A healthy modern car settles to a low standing current. A car whose main relay never opens will sit far above it.

If resting voltage falls noticeably every night, something is awake.

Causes

The relay

Contacts weld closed from switching load, or fail to close from pitting. Relays are cheap.

The swap test applies: exchange with an identical relay from a non-critical circuit: fuses and relays basics for owners.

The control wiring

The module holds the relay coil energised. A chafed wire shorting to ground keeps the relay on permanently, which produces exactly the never-sleeps symptom.

Corrosion in the relay socket

Common in engine bay boxes that have taken water. Look for green oxide and heat discolouration around the socket.

The module driver

The output that controls the relay coil can fail. Considered last, after the relay and wiring are eliminated.

Aftermarket wiring

Alarms, trackers, dashcams and audio installations spliced into a permanent feed are a leading cause of “the battery goes flat” complaints, and they can also keep modules awake.

Aftermarket accessories electrical faults, dashcam installation done right and alarm and tracker installation.

The confusing part

When the module loses power unexpectedly, it does not get to write anything to memory in an orderly way. That produces codes that describe consequences rather than the cause:

• Memory and checksum errors.
• Communication codes from every module that expected the ECM to respond: U0100 lost communication with ECM.
• Sensor codes logged during the moment of power loss.

If you see a P068x code among a crowd of unrelated codes, treat the power relay as the cause and the others as symptoms. Fix the relay, clear everything, and see which codes come back.

Sequence

1. Note the direction — no power, or never sleeps.
2. Feel the relay an hour after parking. Warm means it is still energised.
3. Measure resting current or track voltage over several days.
4. Swap the relay with an identical one.
5. Inspect the socket for corrosion.
6. Check for aftermarket wiring on permanent feeds.
7. Clear all codes and re-scan after the repair.

The practical next step: pair an ELM327 adapter, open the free tier of the app and check resting voltage each morning for a week. A battery that loses a little every night is telling you something never went to sleep.