P0684
PowertrainGlow Plug Control Module to PCM Communication Circuit Range/Performance
On a cold morning you turn the key and the engine churns over for longer than it should before it catches, then runs lumpy until it warms through. The dash warning light comes on too. Behind all that, the engine control module (PCM) and the glow plug control module have stopped talking to each other properly. The PCM expects a sensible status message back from the glow plug module about plug heating, and when that message is missing, garbled, or out of range, it logs P0684. This is a diesel-only fault since petrols don't use glow plugs.
ⓘ Information only. This page provides general educational information about fault code P0684. 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 P0684 mean?
P0684 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Glow Plug Control Module to PCM Communication Circuit Range/Performance.
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 warning light on the dash, sometimes the only thing you notice on a warm day
- • Long cranking before the engine fires, worst first thing in the morning
- • Rough idle or a stumble for the first minute or two until it warms up
- • Occasional no-start in cold weather when the plugs aren't being heated at all
- • Glow plug warning lamp behaving oddly, staying on longer than normal or flashing
Possible causes
Causes commonly associated with P0684, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Faulty glow plug control module, a common failure point on a lot of modern diesels where the module sits low down and cops road salt and water
- 2. Corroded or loose connectors and wiring between the glow plug module and the PCM, often the real culprit and far cheaper than a module
- 3. Weak battery or poor charging keeping the supply voltage to the glow plug module too low to operate or report correctly
- 4. Blown fuse or bad earth on the glow plug module supply circuit
- 5. Damaged or pushed-back pins in the harness connectors, which gives an intermittent fault that comes and goes
- 6. PCM input fault or corrupt calibration on either module, less common and usually the last thing you suspect
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. Read the codes and write down everything that's stored alongside P0684. Individual glow plug circuit faults (P0671 to P0674) or a low-voltage code point you straight at the cause before you touch anything
- 2. Wiggle-test and inspect the connectors at the glow plug module. These modules live in the firing line for road grime, so look for green corrosion, damp, and pins that have backed out of the housing
- 3. Check battery voltage and the supply feeding the glow plug module with a multimeter. You want a healthy resting battery around 12.4-12.7V and a stable feed at the module. A tired battery throws this code all on its own
- 4. Check the fuse and the earth point for the module circuit. A poor earth gives the same symptoms as a dead module and costs nothing to fix
- 5. Clear the code and run it through a few cold starts. If it comes straight back you're chasing a hard fault. If it's intermittent, that's nearly always a connector or wiring problem rather than the module itself
Common questions about P0684
Can I save money with a cheaper aftermarket glow plug module instead of a genuine one? +
Decent aftermarket and reman glow plug modules from a known brand are fine for everyday driving and can save you a fair bit over main-dealer prices. The trap is the no-name eBay units, which sometimes don't report status correctly to the PCM and just set P0684 all over again. If you do go aftermarket, buy from a reputable motor factor and confirm it matches your exact engine code, because these modules vary between variants of the same engine. And before you buy anything, rule out the wiring. Plenty of these get blamed on the module when it's actually a corroded plug or a flat battery.
Is it safe to keep driving like this? +
For short trips in mild weather you'll probably get away with it, since a warm diesel doesn't need the glow plugs much. The risk is cold mornings. With the glow plugs not heating, you can get a no-start or a really rough first minute, and repeated hard cold starts are hard on the battery and starter. It won't leave you stranded on the motorway, but don't ignore it through winter. Sort it before the weather turns.
Will this stop me passing the MOT? +
The code on its own isn't a fail, but a diesel MOT checks the engine management light, and if the warning lamp is lit when the tester looks at it that's a fail under the current rules. Fix the underlying fault, clear the code, and drive it enough cycles to make sure the light stays off before you book it in. A heavily smoking diesel can also struggle on the emissions test, though P0684 itself is about communication rather than combustion.
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 →