P0678

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 patterns: 2003-2007 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 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. 2008-2010 Ford 6.4L Powerstroke commonly throws P0678 from the same UVCH harness chafing. 2001-2016 Duramax 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. 2008-2018 Ford 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).