Powertrain, generic · Computer and auxiliary outputs

P066C Cylinder 2 Glow Plug Control Circuit Low

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. P066C means the circuit to the cylinder 2 glow plug is shorted to ground or draws overcurrent. GM's engine-module drivers flag a circuit impedance to ground below 0.11 Ω (10 of 20 samples at 100 ms) with the check engine and glow plug lamps after two trips; Ford's 2023 Power Stroke checks during heating for more than 22 A for 0.3 s or 20–22 A for 2 s. The channel is switched off, so plug 2 no longer heats – harder cold starts and white smoke follow. A plug shorted internally is the usual cause; a wrong plug type is another.

Likely causes

Glow plug cylinder 2 shorted internally (element against body)
Plug wire insulation damaged and shorted to ground (block, bracket)
Wrong plug type (resistance or voltage rating)
Driver in the glow or engine module faulty, or software (GM 3.0 L 2020)

Symptoms

  • Check engine and glow plug lamp
  • Harder cold start, especially in frost
  • Rough running and white smoke right after a cold start
  • Possibly blown glow system fuse

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

P066DP0672P06BAP06C6P0670P064C

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