Diesel Glow Plugs: Symptoms of Failure and How to Test Them
What glow plugs do in a diesel engine, which symptoms a bad glow plug gives (hard cold starting, white smoke) and how they are tested.
Key points
- Glow plugs heat the chamber to help ignition when the engine is cold.
- Typical symptoms are hard cold starting, white smoke at first and rough running until warm.
- They are not spark plugs: a diesel ignites by compression.
- They are tested by measuring each plug's resistance and current draw, and with a scan tool on modern vehicles.
Unlike a gasoline engine, a diesel has no spark plugs: the fuel ignites from the heat of compressed air. But when the engine is cold, that air loses heat to the walls and may not reach the required temperature. That is where glow plugs come in, heating the chamber so starting is easy and clean.
How do they work?
Each cylinder has a plug with a heating element at the tip that glows red with battery current. When the key is turned, the glow plug light on the dash comes on and the plugs heat for a few seconds. On modern vehicles they can also stay active for a while after starting (after-glow) to reduce smoke and noise while the engine warms up.
Symptoms of failed glow plugs
- Hard starting when cold, especially in low temperatures.
- White smoke on start-up, from fuel that does not burn well.
- Rough running, jerking or vibration until the engine warms up.
- Glow plug light that flashes or stays on, or a check engine light.
- More noise (cold knocking).
In warm climates, a partial failure of the plugs can go unnoticed because the engine starts anyway, and it becomes obvious when the temperature drops.
Why do they fail?
- Cycle wear: thousands of heating cycles eventually exhaust the element.
- Overheating from a glow plug relay that keeps them on too long.
- Carbon deposits from poor combustion. See black, white or blue smoke.
- Relay or control unit faults that do not supply them.
How are they tested?
| Method | What it indicates |
|---|---|
| Measure resistance with a multimeter | A plug with no continuity is broken; values out of range, worn |
| Measure current draw | A defective plug draws less; one shorted, more |
| Scan tool / fault codes | Identifies which cylinder or circuit fails on modern vehicles |
| Check the relay and supply | Rules out faults in the pre-heating circuit |
It is also important to check the battery: a glow plug system demands quite a lot of current. See diesel engine won't start.
Replace one or all?
The most common approach is to replace the whole set if several have failed or if the plugs are many years old, because wear is similar. If only one failed because of an isolated defect, it can be replaced alone. The decision depends on the condition of the others.
What if I replace the plugs and it still fails?
If the engine still starts poorly when cold with the glow plugs in good condition, the cause may lie elsewhere: injectors, fuel system pressure, low compression or sensors. See bad diesel injector symptoms and what is Common Rail.
Frequently asked questions
What are glow plugs for?+
They heat the combustion chamber before and during cold starting. The compressed air of a cold engine does not always reach the temperature needed to ignite diesel, and the glow plug helps it.
How do I know if a glow plug is bad?+
The engine starts with difficulty when cold, gives white smoke at first and runs roughly until warm. A flashing glow plug light or a fault code can also indicate it.
Can I replace only the ones that fail?+
You can, but it is common to replace the whole set because the others have the same wear and often fail soon after. It depends on the condition of each one.
Can glow plugs cause damage if they break?+
Yes. If a plug snaps during removal or in the engine, a piece can fall into the chamber. That is why they should be loosened carefully and, if badly seized, left to a workshop.
Diesel Universal technical team
Laboratorio Diesel Universal S.A.S. Diesel injection laboratory in Barrancabermeja, Colombia. It diagnoses and calibrates injection pumps, injectors and turbochargers on an electronic test bench.
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