When to change spark plugs depends on how the car is driven as much as on the handbook interval. Short, cold, stop and go city trips foul plugs sooner, so rough idle, hesitation, harder starting or worse fuel economy are signs to check them before the scheduled mileage.
Spark plug intervals in a handbook are written for mixed driving with a good share of sustained running. Short, cold, stop-start journeys around the boroughs are the opposite of that, and plugs wear differently as a result.
Why city driving is harder on plugs
A plug needs to reach a temperature at which it burns off deposits. Ten-minute journeys never let it get there, so carbon builds on the electrode instead of being cleaned off. Add frequent cold starts, where the mixture is enriched, and deposits accumulate faster still.
The symptoms of worn plugs
Harder starting, particularly when cold. A rough or uneven idle. Hesitation when you accelerate. Fuel consumption creeping up without any change in how you drive. And eventually a misfire, which will show as a flashing engine light. A flashing light is not the same as a steady one; it means stop, because raw fuel is reaching the catalytic converter.
What happens if they are left
A weak spark makes the engine work harder for the same power, which costs fuel. A genuine misfire sends unburnt fuel into the exhaust, which overheats and destroys the catalytic converter, turning a modest job into an expensive one. It also puts extra load on the ignition coils.
Plugs are not all the same
Copper plugs are inexpensive and short-lived. Platinum and iridium plugs last far longer and cost more. Fitting a cheaper plug than the engine was designed for is a false economy, because the labor to reach them is the same either way and on some engines that labor is significant.
Have the coils checked at the same time
Coils and plugs work together, and a failing coil produces symptoms identical to a worn plug. Replacing plugs without testing the coils is how a misfire returns a week later. See engine diagnostics.
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When to Change Spark Plugs: Questions
How do I know when to change spark plugs?
Signs of when to change spark plugs include rough idle, hesitation when accelerating, harder starting, a misfire light, and falling fuel economy.
What happens if worn plugs are left too long?
Misfires increase, which can damage the ignition coils and send unburned fuel into the catalytic converter.
Should all the plugs be changed together?
Yes. Replacing the full set keeps the cylinders firing evenly.
