P0686 Engine computer power relay: signal or voltage too low (control)

By: Diagno editorial team · Updated: 2026-10-15

P0686 means: Engine computer power relay: signal or voltage too low (control). The fault is in the control module's own memory, processor, supply or internal circuits. Drive gently and have the car checked soon. The repair usually costs about €50–400 in Estonia. The most common cause: connector loose, damp or corroded.

What it means

The fault is in the control module's own memory, processor, supply or internal circuits. Often the real cause is a weak battery or a bad ground. The signal voltage is too low. Usually a wire is broken, a connector is loose or a wire is shorted to ground.

What you may notice

• Engine light on
• The car may go into limp mode
• The engine may not start
• Other modules may also log faults

What could be wrong

1. 1Connector loose, damp or corroded
2. 2Wire broken or shorted to ground
3. 3Faulty control module
4. 4Engine computer fault (rare)

Repair difficulty: Hard

What you can check yourself

First look at the connector and wiring: anything loose, melted or green with corrosion? Check the battery and terminals before anything else. If the fault appeared after a battery change or jump start, clear it and see whether it returns. Best left to a workshop. It needs a diagnostic tool and experience.

How we fix it

1. We read the fault data on the diagnostic tool and check whether the fault is constant
2. We measure the circuit's supply, ground and signal
3. We repair the wire or replace the connector
4. If the wiring is fine, we replace the control module

Standard SAE description: ECM/PCM Power Relay Control Circuit Low