Diesel glow plugs: codes P0670-P0674 and why "cold start" stops working
What glow plugs are, how OBD-II sees their state via P0670, P0671-P0678, why one dead plug ruins the whole cold start, and how much replacement costs on a 4-cyl diesel.
Glow plugs — a unique diesel component. Petrol has spark plugs; diesel has plugs that heat air in the chambers to 850°C before cold starting. Without them a modern diesel won’t start at -15°C. Below: how OBD-II diagnoses glow plugs and what to do.
How glow plugs work
Classic diesel self-ignites from compression (~14:1 → 500°C). But at -20°C compressed air only heats to 400°C — not enough. So each cylinder has a glow plug — a heating coil (like an incandescent bulb) controlled by a relay or glow plug control module (GCM).
Cold-start sequence:
- Key ON — GCM applies 11-12V to plugs for 5-15 sec (pre-glow)
- Plugs heat to 850°C (yellow-red glow)
- “Coil” indicator on dash turns off
- Crank starter — engine catches
- Post-glow — plugs continue heating 30-180 sec after start (to reduce CO/soot)
Key OBD-II codes
| Code | Meaning |
|---|---|
| P0670 | Glow plug control module malfunction |
| P0671 | Glow plug cylinder #1 circuit |
| P0672 | Glow plug cylinder #2 |
| P0673 | Glow plug cylinder #3 |
| P0674 | Glow plug cylinder #4 |
| P0675-P0678 | Cylinders 5-8 (V-engine diesels) |
| P0380 | Circuit A malfunction (old naming) |
Symptoms of bad plugs
- Long cold start — starter cranks 5-10 sec instead of 2-3
- Smoke on cold start — raw grey/white smoke 30 sec after start (unburnt fuel)
- Rough warm-up — 1-2 min uneven idle
- “Coil” indicator flashes — GCM glitch
- Won’t start at < -10°C — critical
Causes and repair
One dead plug
- P0671 (cyl 1) — that specific plug is dead
- Symptom: three cylinders “catch” normally, one stays cold → rough warm-up
- Fix: replace ALL 4 plugs at once (if one died — others are near lifespan). Cost: plugs $40-100 per set + $100-300 labour (access is often hard, need to remove intake)
GCM (glow plug control module) faulty
- P0670 — module itself dead
- Symptom: no plug heats (indicator clears fast but plugs are cold)
- Fix: GCM replacement ($100-350 part + $50-100 labour)
Broken / shorted wire
- Common on 8-10-year-old diesels
- Symptom: code on a specific cylinder, plug tests fine
- Fix: check wire, replace connector ($10-30)
Life expectancy
- OEM NGK / Bosch / Denso — 50-95k mi
- Clones — 20-30k mi (not recommended)
- Ceramic — 125-190k mi, but 3-5x more expensive
Wear sign (no code yet): on cold start, the coil indicator time increases from normal 3-5 sec to 10-15 sec.
What OBD-II can hint
Via Mode 22 (manufacturer PIDs) on VW/Audi/BMW/Mercedes:
- Pre-glow time (sec) — typically 3-5 sec at +20°C, up to 15 sec at -20°C
- Plug current (A) — should be 8-12A per plug
- Plug temperature (where there’s thermal feedback) — 850-950°C in operation
- GCM temperature — monitored, on overheat shuts down plugs for protection
Checklist for a diesel 8+ years old
If you have an old diesel:
- Every 3000 mi — run an OBD scan for P067X
- If cold start takes longer than 5 sec → time to check plugs
- On diesel filter change — ask the mechanic to check plugs (multimeter for resistance, spec 0.6-1.5 Ω)
- In winter — keep tank > 25% (thick fuel in the tank retains temperature)
Our Prediction Agent tracks the “coil indicator time” trend — warns 2-3 weeks before P067X.
Try it on the free tier — pair an ELM327 adapter. P067X codes on every diesel car. Deep glow plug PIDs via Mode 22 — on paid tier.