P0685: ECM/PCM Power Relay Control Circuit/Open

The engine module switches its main relay coil to ground (control circuit), reads the switched voltage back on a sense line and keeps the relay on briefly after ignition off to save learned values. P0685 means the relay control circuit is open: GM measures with the driver off and flags 200 kΩ or more to ground. GM checks with run/crank voltage above 11 V and sets the code after 8 of 10 samples at 250 ms, with the lamp after two trips. GM Europe's 2.0 L turbo compares commanded and actual state for 2 s, Ford's 2023 diesel checks continuously within 0.5 s. A real open keeps the relay from pulling in – no start; on Saab's V6 there is then no communication with the engine module and the MIL flashes. GM notes P0686 may set alongside.

Medium severity. Can I keep driving?It depends on the symptoms. If the car is running normally you can go to a repair shop; if it goes into limp mode, drive as little as possible.

Symptoms

Check engine light
Engine does not start or stalls
No communication with the engine module (Saab)
MIL flashes rapidly, other relays click (Saab)

Most likely causes

Open or corroded control wire between relay coil and module or connector
Relay coil open or relay badly seated
Relay coil supply missing (fuse, battery positive)
Weak battery or voltage dip
Module driver faulty

What to check, in order

Start with the simplest. You can tick off each step.

Check the battery and grounds
Check the continuity of the wire between the component and the control module.
Clear the code and see if it returns
If it is a reference voltage code, disconnect sensors one at a time and see if the voltage recovers
Check whether a software update is available
Do not replace the control module without a full diagnosis

Good to know

Repair difficulty: medium
It affects emissions.

What each character of P0685 means

P: Powertrain: engine and transmission
0: Generic code: same on every make
6: Control modules and outputs
85: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0685?

It depends on the symptoms. If the car is running normally you can go to a repair shop; if it goes into limp mode, drive as little as possible.

What causes code P0685?

The most common causes are: open or corroded control wire between relay coil and module or connector; relay coil open or relay badly seated; relay coil supply missing (fuse, battery positive).

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.