Powertrain, generic · Computer and auxiliary outputs

P0678 Cylinder 8 Glow Plug Circuit/Open

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

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

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

Repair estimate

DifficultyMediumDIY possible
Parts and labour€50–500
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

glow plugglow plug wiringglow plug control module

Related codes

P068EP068FP06C0P06CCP0670P052F

References: law.cornell.edu, gsi.ext.gm.com, fordservicecontent.com, gsi.ext.gm.com.