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) |
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.
