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OBD-II Code

P0679

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–$3,500

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 $3,500 (head pull for seized cyl 8 plug on the 6.0).

P0679 means the ECU detected a glow plug circuit fault on cylinder 8 — open, short, or resistance out of spec from the GPCM monitor.

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 procedureP0679 means the ECU detected a glow plug circuit fault on cylinder 8 — open, short, or resistance out of spec from the GPCM monitor.

P0679 means the ECU detected a glow plug circuit fault on cylinder 8 — open, short, or resistance out of spec from the GPCM monitor.

The cheapest-first diagnostic ladder

  1. Ohm the cyl 8 plug at its terminal — healthy 0.4-2.0 ohms cold (0.6-1.2 typical), 0 shorted, infinite open, 4+ weak.
  2. Confirm GPCM PWM command at 0-100% with glow-on time 5-30 sec scaled to coolant temp.
  3. Amp probe on the cyl 8 feed should show 7-20A initial draw falling to 5-15A; flat 0A with command ON is an open, sustained 20A+ is a short. Cylinder 8 only exists on V8, V10, or V12 diesels — in modern shop work that's a V8 Powerstroke or Duramax — and cyl 8 is universally the rearmost cylinder on one bank, which means the worst access on the engine. 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 P0679 from cylinder 8 (passenger bank, rearmost) — cyl 8 lives under the cowl behind the alternator and is…
  • 2003-2007Ford 6.0L Powerstroke V8 throws P0679 from cylinder 8 (passenger bank, rearmost) — cyl 8 lives under the cowl behind the alternator and is the single hardest plug to service on the engine; if it's seized, this is often the cylinder that pushes the job to a full head pull. Plug-seizure rates on 6.0 cyl 7/8 are high enough that experienced shops quote head removal as a contingency on every P0678/P0679 job.
  • 2008-2010Ford 6.4L Powerstroke throws P0679 from the under-valve-cover harness wear-through at the rear rocker.
  • 2001-2016Duramax V8 throws P0679 on cyl 8 (passenger bank rearmost) — Duramax plug access is better than Powerstroke but the GPCM mounting position on the passenger inner fender means heat-cycle damage to the cyl 8 driver inside the module is also common; if the plug ohms good but amp draw is 0, swap the GPCM.
  • 2008-2018Ford 6.7L Scorpion Powerstroke throws P0679 from harness corrosion.

Estimated repair$150 (single plug with extended labor) to $3,500 (head pull for seized cyl 8 plug on the 6.0).

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