Powertrain, generic · Computer and auxiliary outputs

P066F Cylinder 3 Glow Plug Control Circuit High

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; a faulty channel is switched off. CARB requires glow plugs to be checked for correct current and voltage drop and, since model year 2010, a single plug outside specification to be detected and named. P066F means the circuit to the cylinder 3 glow plug is shorted to battery voltage, so the driver cannot switch it off. GM's engine-module drivers detect an impedance to the supply below 0.5 Ω (0.14 Ω for low-resistance plugs) in 10 of 20 samples, with the lamps after two trips; Ford's 2023 Power Stroke sees 0 A through its measuring path while the plug is commanded on (about 1 s). A permanently powered low-voltage plug can overheat and burn through, after which an open-circuit code follows, and the battery may drain.

Likely causes

Cylinder 3 plug wire touching battery positive (chafing in the harness)
Moisture or corrosion in the module or plug connector (leakage between pins)
Driver in the glow or engine module shorted (permanently on)
Glow module software (GM 3.0 L 2020)

Symptoms

  • Check engine and glow plug lamp
  • Plug permanently powered, possible battery drain
  • Plug burns through, then an open-circuit code
  • Often no symptom when warm

Repair estimate

DifficultyMediumDIY possible
Parts and labour€60–600
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

P066EP0673P06BBP06C7P0670P064C

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