P0684
Glow Plug Control Module to PCM Communication Circuit Range/Performance
- Severity
- Low
- Driving
- Drivable — book it in
The glow plug control module is communicating, but what it reports is not plausible - the engine module cannot reconcile it with what it expects. Often this means the glow plug module is reporting a fault of its own that the engine module cannot interpret.
Possible causes
- CommonFailed glow plug or plugs, reported but not resolvable
- CommonFailed glow plug control module
- CommonWiring fault corrupting the messages
- CommonLow battery voltage
- PossibleSoftware mismatch after a module replacement
What you might notice
- Hard starting when cold
- White smoke on a cold start
- Rough running for the first minute from cold
- Glow plug light behaving oddly
- No symptom at all in warm weather
- Glow plug warning light on
What to check, in order
- Read the glow plug module's own codes if the tool can reach it.
- Check battery voltage and state of charge.
- Test each glow plug individually.
- Ask whether a module was recently replaced - an unconfigured module reports things the engine module cannot interpret.
Read this code from your own car
Plug an ELM327 adapter into the OBD-II port under the dashboard. Pin16 reads every stored, pending and permanent code, explains each one like this page does — offline, with no account — and shows what the car recorded when the fault was set.
Where this code sits
P00xxP01xxP02xxP03xxP04xxP05xxP06xxP07xxP08xxP09xxP0AxxP0BxxP0CxxP0DxxP0ExxP0FxxP10xxP11xxP12xxP13xxP14xxP15xxP16xxP17xxP18xxP19xxP1AxxP1BxxP1CxxP1DxxP1ExxP1FxxP20xxP21xxP22xxP23xxP24xxP25xxP26xxP27xxP28xxP29xxP2AxxP2BxxP2CxxP2DxxP2ExxP2FxxP30xxP31xxP32xxP33xxP34xxP35xxP36xxP37xxP38xxP39xxP3AxxP3BxxP3CxxP3DxxP3ExxP3Fxx
Computer output circuits and control module
P0600P0601P0602P0603P0604P0605P0606P0607P0608P0609P0610P0611P0612P0613P0614P0615P0616P0617P0618P0619P0620P0621P0622P0623P0624P0625P0626P0627P0628P0629P0630P0631P0632P0633P0634P0635P0636P0637P0638P0639P0640P0641P0642P0643P0644P0645P0646P0647P0648P0649P0650P0651P0652P0653P0654P0655P0656P0657P0658P0659P0660P0661P0662P0663P0664P0665P0666P0667P0668P0669P0670P0671P0672P0673P0674P0675P0676P0677P0678P0679P0680P0681P0682P0683P0684P0685P0686P0687P0688P0689P0690P0691P0692P0693P0694P0695P0696P0697P0698P0699