Spark plugs are among the smallest and least expensive components in a petrol engine, yet their condition has a direct bearing on starting reliability, fuel efficiency, engine smoothness, and emissions. At Brookes Auto Repair, spark plug inspection and replacement is a routine part of our service work — and what the condition of a plug reveals about an engine is often more valuable than the cost of the plug itself.
What a Spark Plug Does
The spark plug ignites the compressed air-fuel mixture in each cylinder at precisely the right moment in the engine’s cycle. The quality of this ignition event — its timing, its intensity, and its consistency — determines how completely the fuel burns, how much power is produced, and how efficiently the engine operates. A plug that is firing weakly or inconsistently produces incomplete combustion, which wastes fuel, reduces power, and increases emissions.
How Often Should Spark Plugs Be Replaced?
Replacement intervals vary significantly between vehicles and plug types. Standard copper-core plugs typically last 20,000 to 30,000 miles. Platinum plugs extend this to around 60,000 miles. Iridium plugs — the most durable type, found in many modern vehicles — can last 100,000 miles or more. Your vehicle’s owner’s manual specifies the correct type and interval; using a different type or extending beyond the recommended interval reduces performance and efficiency.
What a Spark Plug Reveals
An experienced technician can diagnose engine problems from plug condition. A plug with heavy black carbon deposits suggests a rich-running engine or oil consumption. A white or light grey deposit pattern on the electrode may indicate a lean mixture or overheating. An oily plug points to oil entering the combustion chamber — a sign of worn piston rings or valve stem seals. These diagnostics are only possible when the plugs are removed and inspected, which is another reason regular servicing provides value beyond the service items themselves.
Misfiring: The Symptom to Watch For
A misfiring engine — characterised by a rough idle, a stumble under acceleration, or a flashing engine management light — is often caused by a failing spark plug or its associated ignition coil. A misfire causes unburned fuel to enter the exhaust system, which can damage the catalytic converter if left unaddressed. Prompt investigation is worthwhile.
For spark plug replacement and ignition system servicing, contact Brookes Auto Repair at brookesautorepair.com.
