P0678: Cylinder 8 Glow Plug Circuit/Open

What does P0678 mean?

Modern diesel glow systems switch each glow plug through its own power semiconductor (in a glow plug module or the engine module), so current and faults can be measured per plug. CARB requires glow plugs to be checked for current and voltage drop and, since model year 2010, a single faulty plug to be detected and named. P0678 means the load circuit of the cylinder 8 glow plug is open while glowing. Thresholds: GM engine-module drivers 200 kΩ or more (10 of 20 samples, two trips), GM's L5P glow module less than 4.0 A, Ford's 2023 6.7 L Power Stroke less than 1 A (cylinders 7 and 8 exist only on the V8). Older GM systems (LB7, LML) report open and short under this code. GM fixed false glow plug codes on 2017–20 heavy-duty diesels by reprogramming the glow module.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• Check engine light (GM also glow plug lamp)
• Harder cold start in low temperatures
• Rough idle and white smoke after a cold start
• Often no symptoms when warm

Common causes

• High Glow plug cylinder 8 burnt out (element open)
• Medium Plug connector loose or corroded, terminal nut loose, wire broken
• Medium Glow module supply or ground (several cylinder codes at once)
• Low Glow module or driver faulty, or software (GM bulletins)

Likelihood is a general estimate across vehicles; your make and model may differ.