Spark plugs ignite the fuel and air mixture in a petrol engine on every combustion stroke. They are a wear item and need periodic replacement, though the interval depends on the type of plug fitted.
Spark plug replacement intervals
Standard copper spark plugs typically need replacing every 20,000 to 30,000 miles. Platinum plugs last around 60,000 miles. Iridium plugs, fitted to many modern engines, can last 80,000 to 100,000 miles. Your vehicle handbook will specify both the correct plug type and the replacement interval for your engine.
Symptoms of worn spark plugs
- Rough idle: The engine vibrates or runs unevenly when sitting still.
- Misfires: A noticeable hesitation or juddering under acceleration.
- Poor fuel economy: Worn plugs fire less efficiently, requiring more fuel to produce the same power.
- Slow or difficult starting: Particularly noticeable in cold weather.
- Engine warning light: A misfire caused by a worn plug will often trigger the engine management light with a P030X misfire fault code.
Emissions and MOT
Worn spark plugs reduce combustion efficiency and increase hydrocarbon emissions. A car with worn plugs is more likely to fail the MOT emissions test, particularly if the plugs are well past their interval. Replacing plugs before an MOT on a high-mileage car is often worthwhile.
Iridium plugs and independent garages
Some drivers assume only main dealers can source the correct iridium plugs for their engine. This is not the case. We source the correct manufacturer-specified plug for every car we service. Call 0131 554 3423 or book a service online.