Spark Plugs: When to Replace Them and Signs They're Worn
Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent
What spark plugs do and why they wear
In a petrol engine, each cylinder has a spark plug that fires thousands of times a minute to ignite the fuel and air. Every spark erodes the electrodes slightly, so the gap widens over time. A wider gap needs more voltage to jump, which strains the ignition coil and makes a weak or missed spark more likely.
Deposits from oil, fuel and short trips also build up on the plug, which can make the spark weaker still. Diesel engines do not use spark plugs, so this guide applies to petrol cars and petrol hybrids.
When to replace spark plugs
There is no single answer, because intervals vary so much between engines. As a general picture:
| Plug type | Typical use | Interval |
|---|---|---|
| Copper core | Older cars and many classics | Relatively short, check the handbook |
| Platinum | Many modern cars | Longer |
| Iridium | Many modern and turbo engines | Often the longest, but may also have a time limit |
The right interval for your car is in the service schedule, or we can look it up from your reg. Many manufacturers set a time limit as well as a mileage, and low mileage cars still need plugs changing on time. Turbocharged petrol engines are often harder on plugs, so do not stretch the interval.
Spark plugs are often part of a full or major service at the right mileage. Our interim vs full service checklist explains what each service usually covers.
Engine problem or belt due?
Timing belts, water pumps, coolant and fault finding done at your door.
WhatsApp for a free quote Call 07563 350335 Contact form
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
Worn spark plug symptoms
- Misfiring: a stutter, jerk or loss of power, especially under load such as climbing hills or overtaking.
- Rough idle: the engine shakes or the rev needle hunts at a standstill.
- Hard starting: more cranking than usual, worse on cold or damp mornings.
- Hesitation: a flat spot when you press the accelerator.
- Worse fuel economy: incomplete combustion wastes fuel.
- Engine management light: often with a misfire code. A flashing light means a misfire serious enough to risk damaging the catalytic converter.
These symptoms are not unique to spark plugs. Ignition coils, injectors, air leaks and sensors can cause the same thing, which is why reading the fault codes matters. See our guides to engine misfire symptoms and coil pack failure symptoms.
Why winter shows up weak plugs
Cold, damp air makes a petrol engine harder to start, and the ignition system has to work harder. A worn plug that coped in summer may cause rough running for the first minute on a frosty morning. Damp also finds weak spots in ignition leads and coil boots, so a misfire that comes and goes with the weather is a useful clue. Short trips around Aylesham and the villages, where the engine never fully warms up, add more deposits to the plugs.
What happens if you leave them too long
- Coils work harder and can fail early.
- Unburnt fuel from misfires can overheat and damage the catalytic converter.
- Fuel consumption goes up.
- Plugs left in for many years can seize or damage the threads in an aluminium cylinder head when removed.
That last point is a good reason to keep to the interval. Removing a seized plug can turn a simple job into a much bigger one.
Should you change them yourself?
On some older engines it is straightforward. On many modern engines, the plugs sit under covers, coils or even the inlet manifold, and they need the correct gap and tightening torque. Cross threading a plug in an aluminium head is an expensive mistake. Only do it if you have the right tools and information, and only on a cold engine.
How we can help
At your home or workplace we can read the fault codes, check plugs, coils and leads, and replace spark plugs to the correct specification, either on their own or as part of a service. Labour is £30 to £50 per hour depending on the job, and we quote from your reg before any work. Send your reg and postcode on WhatsApp 07563 350335 or use our contact page. If the warning light is already on, our engine management light guide explains what to do in the meantime.
Engine problem or belt due?
Message us on WhatsApp with your reg and postcode, or use our contact form, for a price before any work starts.
WhatsApp for a free quote Call 07563 350335 Contact form
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
Frequently asked questions
How often should spark plugs be changed?
It depends on the engine and plug type, so follow the service schedule for your car. Older copper plugs need changing much more often than modern platinum or iridium plugs, which can last a long time but also have a time limit on some cars.
What are the symptoms of worn spark plugs?
Common signs are misfiring, a rough or shaky idle, hesitation when accelerating, harder starting especially in cold or damp weather, worse fuel economy and the engine management light coming on or flashing.
Do diesel cars have spark plugs?
No. Diesel engines ignite fuel by compression and use glow plugs to help cold starting, so spark plug advice only applies to petrol engines, including petrol hybrids.
Is it bad to leave spark plugs too long?
Yes. Worn plugs put extra strain on ignition coils and can cause misfires that damage the catalytic converter, and plugs left in for many years can seize in the cylinder head.
