P0685

ECM/PCM Power Relay Control Circuit/Open

Car Fault Codes Engine computer & electrical

Can you still drive? With care, short trips only

Who can fix it? Needs a workshop

Typical repair time 15 minutes – 4 hours

Typical cost SAR 50 to 5,000

What it means

On many vehicles a main relay powers the engine computer (ECU) and some engine components such as injectors and sensors, and the computer controls this relay. The computer detected an open or fault in the relay control circuit, so the engine may not start or may stall suddenly.

Symptoms4

Engine cranks but does not start, or nothing responds at all.
Sudden stalling while driving.
Check engine light on, possibly with codes in several systems.
Scan tool sometimes unable to communicate with the computer.

Possible causes6

Faulty main relay or a relay not seated firmly in the relay box.
Blown fuse feeding the relay or the computer.
Broken or corroded control circuit wiring or connectors.
Loose or corroded battery terminals or a poor ground.
Moisture inside the relay box.
Faulty control circuit inside the engine computer (rare).

How to diagnose5

Check the battery, its terminals and ground cables first.
Check the fuses related to the main relay and the computer.
Swap in an identical known-good main relay to test.
Measure voltage on the control circuit at the relay with the key on.
Check wiring continuity between the computer and the relay against the manufacturer wiring diagram.

How to fix6

Clean and tighten the battery terminals and grounds.
Replace a blown fuse and find the reason it blew.
Replace the main relay if it is faulty.
Repair damaged wiring or connectors and dry out the relay box.
Replace and program the engine computer only if its control circuit is proven faulty.
Clear the codes and confirm normal starting and that the code does not return.

Repair cost

0–500

500–2,000

2,000–5,000

5,000+

In Saudi riyals; the highlighted part is the expected range for this fault.

The fix is usually a relay, fuse or cleaned terminal at low cost; the upper end applies only if the computer must be replaced.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions

The engine will not start with this code. What do I check first?

Start with the battery and its terminals, then the fuses and the main relay, since these are the most common and cheapest causes.

Can I drive with this code?

With great caution if the engine runs, because losing power to the computer stops the engine suddenly. Repair it as soon as possible.

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This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.

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