Powertrain, generic · Computer and auxiliary outputs

P0684 Glow Plug Control Module 1 to PCM Communication Circuit Range/Performance

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

Glow plug module 1 reports to the engine module over a feedback wire (Bosch-type systems expect it inverse to the activation line) or the bus; GM and Ford report a lost bus link as U0106, Saab as P0380/P1380, and no manufacturer document available to us uses P0683/P0684 themselves. P0684 means communication exists but is implausible: wrong level or duty cycle, implausible content or a feedback that does not match the activation state – as in the Saab logic, where the feedback must be the inverse of the command. The engine module can then no longer rely on the glow system status; individual plug faults may go unreported. Contact resistance at the pins and voltage dips at the glow module during high glow currents are typical causes.

Likely causes

Contact resistance or corrosion at connectors or pins of the communication line (signal distorted)
Supply or ground problem at the glow module (voltage dip at high glow currents)
Glow module faulty, or software/configuration not matching the engine module
Engine module input stage faulty

Symptoms

  • Check engine light (glow plug lamp may flash or stay on)
  • Harder cold start, long cranking in frost
  • Rough running and white smoke after a cold start
  • No individual plug reports any more

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–600
Time0.5–3 h

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

Parts involved

glow plug control moduleglow plug feedback lineengine control module

Related codes

P0683P0670P064CP052FP06FDU0106

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