P0686
PowertrainECM/PCM Power Relay Control Circuit Low
Usually a small job, often a relay or a corroded earth, but it can leave you stranded so don't ignore it. The ECM has spotted that the voltage on the circuit feeding its main power relay is sitting lower than it should be. That relay is what keeps the engine brain powered up, so when its supply drops the car can struggle to start or cut out without warning. Most of the time it's a tired relay or a poor connection rather than the ECM itself.
ⓘ Information only. This page provides general educational information about fault code P0686. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0686 mean?
P0686 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: ECM/PCM Power Relay Control Circuit Low.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine cranks over fine but won't fire up, or starts then dies straight away
- • Random stalling, usually worst at idle or when you back off the throttle
- • Engine warning light on the dash
- • Long cranking before the engine finally catches on a cold morning
- • Flat spots or hesitation when you accelerate
Possible causes
Causes commonly associated with P0686, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Faulty ECM/PCM power relay, the usual culprit. They wear out, the internal contacts pit and the voltage drops across them
- 2. Corroded or loose battery terminals dragging the whole supply voltage down before it ever reaches the relay
- 3. Poor earth or ground point, a classic on older cars where the engine-to-body strap or a bolted earth has corroded
- 4. Damaged or chafed wiring in the relay control circuit, sometimes a chewed loom or a connector full of green crust
- 5. Blown fuse or fusible link in the ECM power feed
- 6. Weak battery that simply can't hold enough voltage to operate the relay properly
- 7. Faulty ECM/PCM itself, the least likely and the most expensive, so leave this until everything else is ruled out
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Load test the battery and check it's holding at least 12.4V at rest. A tired battery throws all sorts of phantom electrical codes, so sort this before you spend a penny elsewhere
- 2. Clean and tighten the battery terminals and check the main earth straps. Wiggle the connections while watching live voltage if you can
- 3. Pull the ECM power relay, inspect the pins for burning or pitting, and swap it with a matching known-good relay from elsewhere in the fusebox
- 4. Check the fuses and any fusible links in the ECM supply circuit for breaks or signs of heat
- 5. With the ignition on, measure voltage at the relay socket. You want to see roughly 12V to 13.5V; anything noticeably lower points at a feed or earth problem
- 6. If voltage, earths, fuses and relay all check out, back-probe the control wire from the ECM and consult the wiring diagram before condemning the module
Common questions about P0686
What am I likely to pay to get this sorted? +
If it turns out to be the relay, you're looking at a cheap part, often £10 to £30, plus maybe an hour's labour, so call it £60 to £120 at an independent garage. A main dealer will charge more for the same job. Where it gets pricey is if the wiring loom or an earth point needs repairing, which can run to a few hundred pounds in diagnostic and labour time. ECM replacement is the worst case and can reach four figures once programming is included, but that's rarely where this code ends up.
How do I work out which of these is actually wrong on my car? +
Start with the battery and the earths because they're free to check and they cause more of these codes than people expect. If the battery is healthy and the terminals are clean, swap the ECM relay with an identical one from the fusebox and see if the fault clears. Still there after that? Measure the voltage at the relay socket with the ignition on. A reading well under 12V tells you the problem is in the feed wiring or a bad earth rather than the relay itself.
Can I have a go at this myself? +
The cheap end of it, yes. Cleaning battery terminals, checking fuses and swapping the relay need basic tools and a bit of patience. There's nothing to clean as such here, it's an electrical supply fault, so it's about checking connections rather than servicing a part. The voltage testing is where a multimeter earns its keep, and if you don't fancy chasing wiring through a loom that's the point to hand it to a garage.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →