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OBD-II Code

P0678

Severity

Not yet reviewed

Nobody on our side has assessed this code yet, so we are not going to guess at how serious it is.

Can you drive it

We cannot say

Treat it as unknown until a shop reads it with live data. If the check-engine light is FLASHING, stop driving as soon as it is safe to — that means an active misfire, whatever the code.

Indicative cost (USD)

$150–$2,000

From this code's diagnostic notes. Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.

$150 (single plug with extended labor) to $2,000 (full set with rear-cylinder access labor).

P0678 means the ECU detected a glow plug circuit fault on cylinder 7 — open, short, or out-of-spec resistance reported by the GPCM.

How a shop works this

Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed.

  1. Note every code that is set, not just this one.

    Codes that appear together usually narrow the cause faster than any one of them read alone.

  2. Read the freeze frame — the conditions the fault was recorded under.

    Cold or warm, idle or load, road speed and engine speed. A fault that only appears cold is a different fault from one that only appears under load, even with the same code on the scanner.

  3. Confirm the fault is current rather than history.

    Clear the code and see whether it returns under those same conditions. A code that does not come back may have been logged during a repair, a flat battery, or a bad connection that has since been disturbed.

  4. Then follow the diagnostic notes further down this page, in the order they are written.

    They are written in the order a shop works this code, cheapest checks first. Where they name a component, inspect its wiring and connector before replacing it.

If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.

Common symptoms

  • Check engine light
Read the full diagnostic procedureP0678 means the ECU detected a glow plug circuit fault on cylinder 7 — open, short, or out-of-spec resistance reported by the GPCM.

P0678 means the ECU detected a glow plug circuit fault on cylinder 7 — open, short, or out-of-spec resistance reported by the GPCM.

The cheapest-first diagnostic ladder

  1. Ohm cyl 7's plug at the terminal — healthy 0.4-2.0 ohms cold (typical 0.6-1.2 ohms), 0 shorted, infinite open, 4+ weak.
  2. Confirm GPCM PWM command at 0-100% duty cycle with glow-on time 5-30 sec scaled to coolant temp.
  3. Amp probe on the cyl 7 feed should show 7-20A initial inrush dropping to 5-15A; 0A with command ON = open, sustained 20A+ = short. Cylinder 7 only exists on V8, V10, or V12 diesels (effectively a V8 Powerstroke or Duramax in modern shop work) — and cyl 7 is typically a deep-back cylinder that's hard to reach. Test resistance INDIVIDUALLY at the glow plug terminal — pulling a shorted plug measures 0 ohms; an open plug measures infinite; a marginally weak plug measures 4+ ohms cold and is the most common cold-start hard-start root cause. Replace as a set on diesel engines — mixed-age plugs cause progressive failure.
Vehicle-specific patterns2003-2007 Ford 6.0L Powerstroke V8 throws P0678 from cylinder 7 (driver's bank, rearmost) — cyl 7 sits under the cowl behind the turbo down-pipe and…
  • 2003-2007Ford 6.0L Powerstroke V8 throws P0678 from cylinder 7 (driver's bank, rearmost) — cyl 7 sits under the cowl behind the turbo down-pipe and the brake booster, making it the worst-access plug on the engine; budget 1.5-2.0 hr labor just to remove it without breaking the body. The 6.0 is also notorious for seizing the rear plugs because exhaust heat and coolant seepage corrode the threads.
  • 2008-2010Ford 6.4L Powerstroke commonly throws P0678 from the same UVCH harness chafing.
  • 2001-2016Duramax V8 throws P0678 on cyl 7 (driver's bank rearmost) — Duramax glow plugs are easier to access than Powerstroke because the V-angle is wider, but cyl 7 still requires removing the inner fender liner for tool clearance on some configs.
  • 2008-2018Ford 6.7L Scorpion Powerstroke uses a different (and more reliable) glow system but still throws P0678 occasionally from harness corrosion at the firewall pass-through.

Estimated repair$150 (single plug with extended labor) to $2,000 (full set with rear-cylinder access labor).

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