Spark plugs ignite the air–fuel mixture in petrol engines. They wear gradually, and the spark weakens over time.
Diesel engines do not use spark plugs; they use glow plugs to aid cold starting.
Replacement intervals
Copper plugs might need changing every 20,000–30,000 miles, while platinum and iridium plugs can last 60,000–100,000 miles or more. Always follow the service schedule for your car.
Signs of worn plugs
- ▸Rough idle or engine misfire
- ▸Hard starting
- ▸Poor acceleration
- ▸Worse fuel economy
- ▸Engine warning light, often with misfire codes
DIY or garage?
On many four-cylinder engines, plugs are easy to reach with a plug socket. On some V6 engines, rear plugs need parts removed. Torque settings matter; overtightening can damage threads in the cylinder head.
Coil packs
Modern engines often have individual ignition coils on top of each plug. Misfires can come from a faulty coil rather than the plug, so a diagnostic check helps.
CAUTION: This is general automotive information, not professional mechanical advice. Always follow your vehicle manufacturer’s specifications and service schedule, and have brakes, steering, tyres, and other safety-critical systems inspected by a qualified mechanic.