P0685: ECM/PCM Power Relay Control Circuit/Open
Quick Answer
Not safe to drive
P0685 means the engine computer (ECM/PCM) monitors the circuit that switches its main power relay and found it open or without the expected voltage. On many cars the relay feeds power to the computer, injectors and ignition, so when it does not close the car often will not start. The fault can be the relay, its fuse, a corroded socket, damaged wiring, or low battery voltage, and only rarely the computer itself.
- Severity
- High · Electrical
- Estimated repair cost
- From €22 to €1200+, varies by vehicle
€22 or €1200, depending on your car.
- Replace blown ECM fuse after finding the short€22–€95
- Replace ECM/PCM with programming€350–€1200
A scan shows whether P0685 is really stored on your car, which codes sit next to it and how urgent each one is. You can do this parked, with the ignition on.
A scan needs a Bluetooth OBD2 adapter (Bluetooth LE on iPhone), about €25-60; the built-in demo runs without one. See which adapter to buy
What does P0685 mean?
P0685 is a electrical-related diagnostic trouble code classified as high severity.
What are the symptoms of P0685?
- Engine will not start, or cranks but does not run
- Engine stalls and then restarts after a wait
- Check engine light on
- Limp mode or reduced power if the engine does start
- Instrument cluster and other systems behaving normally while the engine will not fire
- Starts sometimes and not others, depending on the temperature or a bump in the road
What causes P0685?
Worn or failed ECM/PCM power relay
: high
Blown fuse or fusible link in the engine computer's supply
: high
Corroded or loose relay socket, or water in the relay carrier
: medium
Open or shorted wire between the relay, the fuse box and the engine computer
: medium
Weak battery or low voltage during cranking
: medium
Internal fault in the ECM/PCM relay driver, which is uncommon
: low
Is it safe to drive with P0685?
No - address this immediately. The engine computer's main power relay is not switching on as it should. The car may not start or may cut out while driving, and a car that stalls loses power steering assist and brake booster help. Do not drive it if it has stalled or is running badly. A car that starts normally can be driven slowly to a workshop, but expect it to fail without warning.
How do you diagnose P0685?
- Test battery voltage at rest and while cranking before anything else. Low voltage can switch the relay off or make the computer reset.
- Find the engine computer relay and its fuses in the wiring diagram. Remove the relay and check the socket for green or white corrosion, bent contacts and water marks.
- Swap in a known-good relay of the same type and test again. If the code has gone and the car starts, the relay was the fault.
- Measure the relay coil supply and the control side with the ignition on. The computer pulls the control wire to ground when it wants the relay closed.
- Check the fuse with a meter, not by eye. If it blows again at once, there is a short that must be found before another fuse is fitted.
- Look for wiring damage between the relay carrier and the engine computer connector, especially near moving parts, harness bends and where water can sit.
- Check power and ground pins at the engine computer connector under load.
- Replace the computer only after relay, fuse, wiring and ground have been proven good.
How much does P0685 cost to fix?
| Repair | Parts Cost | Labor Cost | Total Estimate | DIY Difficulty |
|---|---|---|---|---|
| Replace ECM/PCM power relay | €30–€100 | €10–€40 | €40–€140 | Easy |
| Replace blown ECM fuse after finding the short | €2–€15 | €20–€80 | €22–€95 | Moderate |
| Repair relay socket or wiring to the ECM | €10–€60 | €80–€250 | €90–€310 | Moderate |
| Battery replacement | €60–€200 | €20–€60 | €80–€260 | Easy |
| Replace ECM/PCM with programming | €250–€900 | €100–€300 | €350–€1200 | Professional |
Prices estimated as of October 2026. Costs vary by region, vehicle, and shop.
P0685 on specific vehicles
P0685 on Volkswagen
VW Jetta 2.5 (North American market) and City Golf
P0685 on Saab
Saab 9-3 (9440)
Related diagnostic codes
Common questions about P0685
- What does P0685 mean?
- It means the engine computer cannot confirm its own power relay is switching on. The relay supplies the computer, injectors and ignition on many cars, so a fault often shows up as a car that will not start.
- Can a bad relay cause P0685 on its own?
- Yes, and it is one of the first things to test. A worn relay or a corroded socket is much cheaper to fix than the computer, so swap the relay with a known-good one and look at the socket before spending more.
- Why does the car start sometimes and not other times?
- A relay or connector that is partly worn can make contact on some starts and not others. Temperature, vibration and moisture change the contact, which is why the fault can seem random.
- Is P0685 the same as P068A?
- No. P0685 is about the relay circuit failing to switch on as commanded, which tends to cause no-start. P068A is about the relay dropping out too early. They share the same relay system, so one fix, such as a weak battery or a bad socket, can sometimes remove both.
- Is P0685 a Volkswagen problem only?
- The code is generic and appears on many makes. Ross-Tech's wiki lists it for VW cars as an ECM relay (J271) circuit open, and Saab's service information lists it as main relay circuit performance. Wording and the relay location differ between makers.
Read our detailed guides
Sources
- wiki.ross-tech.com wiki.ross-tech.com/wiki/index.php/17069/P0685
- saabwisonline.com saabwisonline.com/9-3-9440/2012/dtcs/e39/p0685-00