Key Differences
| Feature | Spark Plugs | Glow Plugs |
|---|---|---|
| Engine Type | Petrol only | Diesel only |
| Function | Creates a spark to ignite fuel | Heats the combustion chamber for cold starts |
| When Active | Every combustion cycle | Pre-start and cold running only |
| Lifespan | 30,000–100,000 miles | 60,000–100,000 miles |
| Failure Symptom | Misfires, rough idle | Hard cold starting, white smoke |
| DIY Difficulty | Easy–Medium | Medium–Hard (risk of snapping) |
Recommended Spark Plugs for vs Glow Plugs
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How Diesel Ignition Works
Diesel engines don’t need a spark — the fuel ignites from compression heat alone. However, when the engine is cold, the compressed air may not reach high enough temperatures. Glow plugs solve this by pre-heating the combustion chamber before and during starting.
Frequently Asked Questions
Does my car have spark plugs or glow plugs?
Petrol engines have spark plugs. Diesel engines have glow plugs. Hybrid vehicles with a petrol engine have spark plugs. Electric vehicles have neither.
Can I change glow plugs myself?
It’s possible but riskier than spark plugs. Glow plugs can seize in the cylinder head due to heat cycling, and snapping one off requires specialist extraction. If they haven’t been changed recently, consider a professional.
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How we check fitment for this guide
We cross-reference fitment figures against manufacturer specifications and OE-supplier catalogue data where available, and matched to the standard codes that actually govern fit — battery type/group sizes, wiper lengths and fitting types, bulb fitment codes, tyre size markings, and oil specifications. We work by make, model, year, engine, and build date, because a single model can take different parts across its life. Where the right part depends on your exact vehicle, we tell you to confirm by registration or VIN. More on our method →