P0670 — Glow Plug Control Module Circuit

Category

Powertrain → Computer Output Circuit

What it means (plain English)

The glow plug control module manages when and how long the glow plugs heat up during cold starts. A P0670 code means the ECM has detected a fault in the glow plug control circuit itself — not just one plug, but the system that powers and times them. Without proper operation, the engine may struggle or fail to start in cold conditions.

Symptoms

  • Check Engine Light (MIL) illuminated
  • Hard starting in cold weather
  • Rough idle after cold start
  • Excessive white smoke (unburned fuel) during startup
  • Increased cranking time
  • In some cases, engine may not start in very low temps

Priority Level

Medium–High —
While it may not cause immediate engine damage, a faulty glow plug circuit can prevent the truck from starting in cold weather and may increase emissions and fuel wash into the cylinders.

Common Causes

  • Failed glow plug control module (most common)
  • Open/short in wiring to the control module
  • Poor battery voltage or weak connections
  • Faulty relay inside the glow plug module
  • Corroded or loose electrical connectors
  • In rare cases, ECM driver fault

How Pros Diagnose It (step-by-step)

  1. Scan tool confirmation — verify P0670 and check for related glow plug codes (P0671–P0678 for individual cylinders).
  2. Visual inspection — check wiring and connectors for corrosion or heat damage.
  3. Battery/voltage check — ensure proper supply voltage to the glow plug module.
  4. Check glow plug resistance — typically 0.5–2 ohms per plug; open or high resistance = bad plug.
  5. Test module output — backprobe with a DVOM or test light during preheat cycle.
  6. Relay check — some systems use internal relays inside the module that may fail.
  7. If no power output and wiring checks out — replace module.

Likely Fixes

  • Replace glow plug control module
  • Repair/replace damaged wiring or connectors
  • Clean corroded battery/ground connections
  • Replace faulty glow plugs (if companion codes are present)
  • In rare cases, reflash or replace ECM if module and wiring are good

Related/Companion Codes

  • P0671–P0678 — Glow Plug Circuit Malfunctions (cylinder-specific)
  • P0380 — Glow Plug/Heater Circuit “A”
  • P1391/P1393 — Manufacturer-specific glow plug heater circuit faults

Tech Notes (Quick Hits)

  • GM Duramax (LMM, LML, L5P) trucks commonly see P0670 due to a failed glow plug module; GM even issued service bulletins for certain years.
  • Ford Powerstroke 6.0L & 6.4L: harness corrosion is a frequent cause.
  • Cummins ISB/ISC: often triggered by poor battery voltage or failing relays inside the module.
  • Always check individual plug codes (P0671–P0678) — P0670 means the system, while those mean the plugs.

For Your Page (Copy Block)

Severity: Medium–High — Repair Soon
A P0670 code means the glow plug control system isn’t working right. Without working glow plugs, your diesel may be hard to start or fail to start in cold weather. Fixing the module or wiring ensures reliable cold starts and prevents extra fuel wash or smoke at startup.

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