P0686: ECM/PCM Power Relay Control Circuit Low

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. P0686 means the control line is shorted to ground: with the driver off it should carry battery voltage through the coil, but it sits at ground (0.5 Ω or less). The relay then stays energised and the module can no longer switch it off – after-run and shutdown are disturbed and the battery can drain. 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. The former obdex title (the wording of P0685) was a shifted entry; P0686 is the 'low' code.

Symptoms

• Check engine light
• Relay stays on after ignition off, battery drains
• Possibly P068B as well

Most likely causes

• Control wire between relay and module chafed to ground
• Relay shorted internally or water in the relay carrier
• Module driver shorted internally

What to check, in order

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

1. Check the battery and grounds
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Clear the code and see if it returns
4. If it is a reference voltage code, disconnect sensors one at a time and see if the voltage recovers
5. Check whether a software update is available
6. Do not replace the control module without a full diagnosis

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P0686 means

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

Frequently asked questions

Can I keep driving with code P0686?

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 P0686?

The most common causes are: control wire between relay and module chafed to ground; relay shorted internally or water in the relay carrier; module driver shorted internally.

Related codes

• P0685ECM/PCM Power Relay Control Circuit/Open
• P0687ECM/PCM Power Relay Control Circuit High
• P068BECM/PCM Power Relay De-Energized Performance Too Late
• P0690ECM/PCM Power Relay Sense Circuit High
• P0689ECM/PCM Power Relay Sense Circuit Low

Sources

• law.cornell.edu: 13 CCR 1968.2
• gsi.ext.gm.com: 25OBDG03A
• gsi.ext.gm.com: 19OBDG03B Diagnostics
• saabwisonline.com: dtc p0685 p0686 p0687 p0689 or p0690

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.