Ignition
Glow Plug
Glow plugs pre-heat the combustion chamber of a diesel engine for cold starts.
Free guide
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Ignition flow
Interactive — tap a partTap a part to see how it fits in the system.
What it does
- Heats the air in the cylinders before a cold start
- Improves cold idle and reduces white smoke
- Stays active briefly after start
When it fails
- Hard cold starting
- White-grey smoke when cold
- Glow plug warning light flashing
- Rough cold idle
How to diagnose
- Measure each plug resistance (a good plug reads a few ohms)
- Check the glow plug relay and fuse
- Test the glow plug controller
Maintenance
- Replace worn glow plugs in a set
- Use the correct heat range
- Never force a stuck plug — they break
Use the diagnosis order. When a part fails, confirm each hypothesis with measurements before replacing anything — codes and symptoms overlap.