Change Car Spark Plugs
Spark plugs produce the spark that ignites the air/fuel mixture in petrol engines. Worn plugs lead to misfires, a loss of power, higher fuel consumption and can trigger a warning light on the dashboard.
What is the role of the spark plugs?
On a signal from the engine ECU, the plug produces a spark at the precise moment the air/fuel mixture should ignite in the combustion chamber. A failing plug generates an EOBD fault code (generally P0301 to P0304 depending on the cylinder concerned) and can light the engine warning lamp.
When should you change the plugs?
The standard interval is 60,000 km (or 45,000 km in severe use). On vehicles fitted with long-life plugs (titanium, platinum, iridium), replacement can extend to 100,000 to 150,000 km. Refer to your vehicle's service book.
Symptoms of failing plugs
These signs should alert you and prompt a check:
Difficult starting
- The engine struggles to start, especially from cold
Misfires
- Shaking, hesitation, fault code P030x
Loss of power
- The engine lacks response under acceleration
High fuel use
- Incomplete combustion raises consumption
Replacement: what you need to know
Difficulty 2/5
Technically easy but needs a spark-plug socket and a torque wrench. The plugs are usually accessible, except on some engines where an air filter housing is in the way.
Cold engine essential
Always wait until the engine is completely cold. Before unscrewing, blow out the plug well with compressed air to avoid dropping debris into the combustion chamber.
Tighten to torque
Screw the plug in by hand until it contacts, then tighten to the recommended torque with a torque wrench. Do not grease the thread.
Price
From £3 to £25 per plug from a motor factor. The average garage price for a full set is around £90. Avoid cheap plugs of doubtful origin.
