ECM/PCM Power Relay Control Circuit Low
What it means
The engine computer (ECU) detected lower than expected voltage on the control circuit of the main relay that powers it and parts of the engine. This usually comes from a weak battery, a control wire shorted to ground or a damaged relay coil.
Related lights
Dashboard lights that may come on with this code.
Symptoms4
- Hard starting or no start.
- Intermittent stalling while driving.
- Check engine light on.
- Repeatedly blown circuit fuse at times.
Possible causes5
- Weak battery or low voltage while running.
- Control circuit wire shorted to the body or ground.
- Internally damaged main relay coil.
- Corrosion or moisture in connectors or the relay box.
- Faulty control circuit inside the engine computer (rare).
How to diagnose5
- Check battery voltage while running and the condition of the terminals.
- Swap in an identical known-good main relay to test.
- Check the fuse, and if it is blown, find the short to ground before replacing it.
- Test the control wire's isolation from ground with both ends disconnected.
- Inspect the harness for chafed insulation or pinched wires.
How to fix5
- Charge or replace a weak battery.
- Replace the main relay if its coil is damaged.
- Repair the wire shorted to ground and re-insulate and secure it.
- Clean moisture and corrosion from the connectors and relay box.
- Clear the codes and confirm normal starting and that the code does not return.
Repair cost
In Saudi riyals; the highlighted part is the expected range for this fault.
In the Saudi market: SAR 50–5,000 (≈ USD 13–1,330)
Our prices come from the Saudi market, converted at a fixed rate (SAR 3.75 per USD). Prices in your country may differ a lot, so we show the cost level.
The cost is low for a relay or wire repair, and rises if the battery must be replaced or the computer is proven faulty.
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
Can the battery cause this code?
Yes. Low battery voltage while running can make the computer read low voltage on the relay circuit, so the battery is checked first.
How is it different from P0685?
P0685 indicates an open or general circuit fault, while P0686 specifies low voltage, pointing the diagnosis toward a short to ground or weak supply.
Was this explanation helpful?
Tap the stars to rate; you can change it later.
What was wrong? (optional)
We never store your IP address, and your comment is not published. Details: Privacy policy · Terms and conditions
Report a mistake
Spotted something inaccurate? Tell us and we will review and correct it.
Report received — thank you
This is your private tracking link. It shows the status and our reply as soon as it is posted.
The link is saved in this browser only.
