P0671–P0678 — Glow Plug Circuit Malfunctions (Cylinders 1–8)

Category

Powertrain → Computer & Output Circuit

What it means (plain English)

Each code from P0671 through P0678 refers to a fault in the glow plug circuit for a specific cylinder:

  • P0671 → Cylinder 1
  • P0672 → Cylinder 2
  • P0673 → Cylinder 3
  • P0674 → Cylinder 4
  • P0675 → Cylinder 5
  • P0676 → Cylinder 6
  • P0677 → Cylinder 7
  • P0678 → Cylinder 8

When one of these sets, the ECM has detected that the individual glow plug for that cylinder isn’t drawing the correct amount of current or isn’t heating properly. Glow plugs help preheat combustion chambers for cold starts in diesel engines.

Symptoms

  • MIL (Check Engine Light) illuminated
  • Hard starting in cold weather (especially as multiple plugs fail)
  • Rough idle or misfire after cold start
  • Excessive white smoke on startup (unburned fuel)
  • Increased cranking time
  • In warmer temps, the truck may start normally with no obvious drivability issues

Priority Level

Medium —
A single bad glow plug often won’t disable the truck, but multiple failures make cold starts difficult or impossible. Over time, poor cold combustion can wash fuel into cylinders and increase emissions.

Common Causes

  • Burned-out or open glow plug (most common)
  • High resistance in glow plug element
  • Damaged wiring harness to glow plug
  • Corroded or loose glow plug connector
  • Glow plug control module not delivering voltage (sometimes misinterpreted as individual fault)

How Pros Diagnose It (step-by-step)

  1. Confirm code(s) — note which cylinder(s) are affected.
  2. Visual inspection — check wiring and connectors to glow plugs.
  3. Resistance test — remove connector and measure glow plug resistance (usually 0.5–2 ohms). Open circuit = failed plug.
  4. Current draw test with an amp clamp — compare cylinder to cylinder.
  5. Check module output — ensure voltage is being delivered during preheat cycle.
  6. If multiple plugs are flagged, suspect the module or harness rather than all plugs failing simultaneously.

Likely Fixes

  • Replace faulty glow plug(s) for the affected cylinder(s)
  • Repair wiring harness/connectors
  • Replace glow plug control module if multiple plugs aren’t powered
  • Clean or reseat electrical connections
  • In rare cases, update ECM software

Related/Companion Codes

  • P0670 — Glow Plug Control Module Circuit (system-wide)
  • P0380 — Glow Plug/Heater Circuit “A”
  • P1391, P1393 — Manufacturer-specific glow plug heater faults

Tech Notes (Quick Hits)

  • On GM Duramax engines, it’s common for individual glow plugs to fail one at a time, especially in cold climates.
  • Ford Powerstroke 6.0L/6.4L often show multiple glow plug circuit codes when the harness or module fails.
  • Cummins ISB/ISC usually trigger individual codes when a plug burns out, but if all 6 are flagged, it’s usually the module.
  • Always verify resistance — a single plug is cheap to replace compared to a module.

Severity: Medium — Repair Soon
Codes P0671–P0678 mean one or more glow plugs aren’t heating properly. That can make cold starts rough, smoky, or even impossible in freezing weather. Replacing a single plug is usually quick and inexpensive, but ignoring multiple failures can strain your engine and leave you stranded.

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