What Is a Spark Plugs?
A spark plug is a precision electrical component threaded into each cylinder head of a petrol engine. It consists of a central electrode and a ground electrode separated by a small gap. When the ignition coil delivers a high-voltage pulse, an electrical arc jumps across this gap, igniting the compressed air-fuel mixture in the cylinder at precisely the right moment in the engine cycle. Modern engines use iridium or platinum-tipped plugs for extended service life.
What Does the Spark Plugs Do?
Without a correctly functioning spark plug, a cylinder cannot fire. The spark must occur at exactly the right moment in the compression cycle — too early or too late causes knocking, loss of power and poor fuel economy. Spark plugs also act as a diagnostic window into the health of each cylinder — the condition of a used plug reveals a great deal about combustion quality, oil burning and mixture ratios.
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Get My Free Fault Scan →Symptoms of a Failing Spark Plugs
Worn or fouled spark plugs cause one or more cylinders to misfire. Symptoms include rough idling, hesitation under acceleration, reduced fuel economy, increased exhaust emissions, a noticeable loss of power and an engine management light showing a cylinder misfire code. Hard starting — particularly in cold weather — is also a common symptom of worn plugs.
When Should the Spark Plugs Be Replaced?
Standard copper spark plugs require replacement every 20,000 to 30,000 miles. Iridium and platinum plugs extend this interval to 60,000 to 100,000 miles on many modern engines. Always replace the full set rather than individual plugs. If coilpacks are also being replaced, change the plugs at the same time as worn plugs cause premature coilpack failure.
If you are experiencing symptoms related to your spark plugs and are unsure whether it needs replacing, a free MotorLoom fault scan can help identify whether the component is likely at fault - giving you the most likely cause and estimated repair cost before you visit a garage.
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Related Parts
Other components that work alongside or are related to the spark plugs: