P0688
PowertrainECM/PCM Power Relay Sense Circuit Open
Most people meet this code after the car suddenly cuts out, refuses to start, or starts dropping into limp mode for no obvious reason. What's happening is that the ECM watches the relay that feeds it power, and it expects to see a healthy voltage signal back from that relay. When that sense wire goes open circuit or the voltage isn't what the ECM expects, it flags P0688. It points at the power supply side of the engine computer rather than the computer being broken itself, which is the part people get wrong.
ⓘ Information only. This page provides general educational information about fault code P0688. 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 P0688 mean?
P0688 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: ECM/PCM Power Relay Sense Circuit Open.
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:
- • Check engine light on, sometimes the only warning before things get worse
- • Hard starting or a no-start where the engine cranks happily but never fires
- • Random stalling while driving, often with no warning and sometimes restarting fine after a few minutes
- • Rough running, hesitation, or the car dropping into limp mode
- • Scan tool struggling to connect or losing communication with the ECM mid-session
- • On some cars the warning light stays off even though the code is stored in memory
Possible causes
Causes commonly associated with P0688, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. A failing ECM power relay, the usual culprit and the cheapest thing to rule out first
- 2. Corroded or broken wiring in the relay sense circuit, often where the loom passes near heat or vibration
- 3. Loose, green, or corroded connector pins at the relay or the ECM plug, very common on older cars and anything that's seen damp
- 4. A blown fuse or tired fusible link in the supply circuit
- 5. Weak or undercharged battery dragging system voltage down below what the ECM wants to see
- 6. A poor earth, a loose ground strap or a corroded chassis bolt upsetting the voltage the module reads
- 7. Internal fault inside the ECM itself, rare and the last thing you should suspect, not the first
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. Pull all stored and pending codes with a scan tool, note them, then clear and drive a short loop to see if P0688 comes straight back. An intermittent that won't return tells you a lot.
- 2. Check battery voltage with everything off. Anything under 12.0V can throw this code on its own, so sort a weak battery before you chase wiring.
- 3. Pull the ECM power relay, check it for sticking or burnt contacts, and swap it with an identical relay from elsewhere in the box if your fuse layout allows. This is a five-minute test that solves a fair share of these.
- 4. Go over the related fuses, the supply wiring, and every connector you can reach, looking for chafing, green corrosion, or a plug that isn't fully home.
- 5. Backprobe the sense wire between the relay and the ECM connector and measure for continuity. An open circuit or high resistance here is your fault, and waggling the loom while watching the meter often finds an intermittent break.
- 6. If the relay, wiring, earths, and battery all test clean, only then start looking at the ECM itself, which usually means a specialist.
Common questions about P0688
How long is this going to take to sort? +
If it turns out to be the relay or a fuse, a competent garage will have it done inside an hour, and you could do the relay yourself in ten minutes once you've found which one it is. Tracing a broken sense wire is the time killer. An intermittent fault that only shows up over bumps can swallow two or three hours of diagnostic labour before anyone even touches a part, so expect to pay for the hunt as much as the fix.
Should I buy a cheap relay or pay for a genuine one? +
For the power relay, a quality branded part like Bosch, Hella, or Tyco is fine and costs a few pounds, no need to pay main dealer prices for a relay. Where I'd avoid the bargain bin is the no-name eBay relays that look right but have weak contacts that fail again in months. If the fix turns out to be the ECM itself, that's a different story and you want a properly programmed unit matched to your car, not a gamble from a breaker.
Can I keep driving it like this? +
I wouldn't. The whole problem with P0688 is that it can stall the engine without warning or leave you with a no-start in a car park, and a power fault to the engine computer is not something that gets better on its own. If it's stalling at junctions or on the motorway that's a genuine safety risk. Get it looked at before you find yourself stranded somewhere awkward.
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 →