Spark Plugs: Signs They Need Changing
Spark plugs ignite the fuel-air mixture in a petrol engine, and worn ones cause rough running, misfires, hard starting and poorer fuel economy. Signs they need changing include a rough idle, hesitation, a flashing engine light or worse mpg. Change intervals vary widely by car, so check your handbook.
What spark plugs do
In a petrol engine, the spark plugs provide the spark that ignites the compressed fuel-air mixture in each cylinder, thousands of times a minute. That controlled ignition is what makes the engine run, so the plugs are fundamental to smooth running.
Over time the plugs wear: the electrode erodes and deposits build up, so the spark gets weaker or less reliable. That is when running problems and inefficiency creep in. (Diesel engines work differently and use glow plugs to aid cold starting, not spark plugs.)
Signs your plugs are worn
Worn or fouled spark plugs show up in how the engine runs.
- A rough or lumpy idle, or the engine feeling like it stumbles.
- Hesitation or a lack of power when accelerating.
- Hard starting, especially when cold.
- Worse fuel economy than usual.
- A misfire, sometimes shown by a flashing engine light.
Why a misfire matters
A misfire, where a cylinder fails to fire properly, is one of the more serious symptoms of worn plugs, and a flashing engine management light usually indicates one. It is worth taking seriously.
An ongoing misfire wastes fuel, makes the car run roughly, and can pump unburnt fuel into the exhaust, which can damage the expensive catalytic converter. So if the engine light flashes or the car runs noticeably rough, do not ignore it; get it checked promptly.
How often to change them
There is no single figure, as it depends heavily on the plug type and engine. Some plugs are designed to last a long time, while others need changing more often, so your handbook is the authority for your car's interval.
Spark plugs are usually changed as part of a service at the recommended interval. Changing them on schedule keeps the engine running smoothly and efficiently, rather than waiting for symptoms to appear and economy to suffer.
Worn plugs and fuel economy
One of the most overlooked effects of tired spark plugs is the slow creep in fuel consumption. As the spark weakens, combustion becomes less efficient, so the engine burns more fuel for the same work, often without an obvious symptom.
Because it happens gradually, many drivers do not notice until plugs are changed and economy improves. Given fuel prices, keeping the plugs in good order is a small cost that can pay for itself in better mpg over time.
Getting them checked
If your engine is running roughly, hesitating, hard to start or drinking more fuel, worn spark plugs are a common cause worth checking, though a misfire can have other causes too, such as ignition coils.
We can inspect and replace spark plugs as part of a service, and diagnose a misfire or rough-running fault to find the real cause. Fresh plugs at the right interval keep a petrol engine smooth, willing and economical.
Frequently Asked Questions
What are the signs of worn spark plugs?
A rough or lumpy idle, hesitation or lack of power, hard starting (especially cold), worse fuel economy, and misfires sometimes shown by a flashing engine light. These point to plugs whose spark has weakened with wear and deposits, and which are due for replacement.
How often should spark plugs be changed?
It varies widely by plug type and engine, so check your handbook for your car's interval. Some plugs last a long time, others need changing more often. They are usually replaced as part of a service at the recommended interval to keep the engine smooth and efficient.
Can worn spark plugs cause a misfire?
Yes. Worn or fouled plugs are a common cause of misfires, where a cylinder fails to fire properly, often shown by a flashing engine light. A misfire wastes fuel and can damage the catalytic converter by passing unburnt fuel into the exhaust, so it should be checked promptly.
Do worn spark plugs affect fuel economy?
Yes, often gradually. As the spark weakens, combustion becomes less efficient, so the engine burns more fuel for the same work, frequently without an obvious symptom. Many drivers only notice the improvement after the plugs are changed, so fresh plugs can pay back in better mpg.
Do diesel cars have spark plugs?
No. Diesel engines ignite the fuel by compression rather than a spark, so they have no spark plugs. They do have glow plugs, which warm the combustion chamber to help cold starting. A failing glow plug makes a diesel hard to start in cold weather and may light a warning lamp.
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Hero image: “1967 Champion Spark Plug Advertisement Readers Digest June 1967” by SenseiAlan (source), licensed under CC BY.