How to Replace Spark Plugs Yourself on a Petrol Engine

A garage will usually charge somewhere between 100 and 250 for a spark plug change on a four cylinder engine, and most of that is labour. The plugs themselves often cost 8 to 20 each. On a lot of everyday cars, the job takes 45 minutes with a socket set, a gap tool and a torque wrench you may already own.

This guide covers how to replace spark plugs yourself on a typical petrol engine: when they actually need changing, which plugs to buy, the order of work, and the two mistakes that turn a cheap job into a cracked coil pack or a stripped cylinder head thread. I have done this on everything from a 1.2 litre city car to a V6 that needed the intake manifold off, and the basics never change.

When Spark Plugs Really Need Changing

Service intervals vary more than people expect. Older copper plugs were often due every 20,000 to 30,000 miles. Modern iridium and platinum plugs commonly run 60,000 to 100,000 miles, and some manufacturers quote even longer. Check your handbook rather than a generic chart.

Symptoms that point to worn plugs include a rough idle, hesitation when you press the accelerator, harder cold starts and fuel economy that has slowly dropped by a few miles per gallon. A flashing engine warning light with shaking usually means a misfire. If that is happening, read our guide to diagnosing a misfire without a scanner first, because a failing coil or injector can look exactly like a bad plug.

Do Not Just Buy the Most Expensive Plug

A common piece of forum advice says to upgrade to the priciest iridium plug you can find. I disagree. The right plug is the one your engine was designed for, with the correct heat range, thread reach and gap.

Fitting a plug with the wrong heat range can cause fouling or, at the other extreme, pre ignition. If your engine came with platinum plugs, iridium is a safe like for like upgrade. If it came with copper plugs, as many older engines did, fancy plugs often bring no measurable gain. Look up the part number from the manufacturer, or use your registration or VIN with a parts supplier, so the plug matches the exact engine code rather than just the model name.

Tools and Parts You Will Need

A ratchet with a spark plug socket, usually 14 mm, 16 mm or 5/8 inch, ideally with a rubber insert or magnet to hold the plug
Extensions of 75 mm and 150 mm, plus a universal joint for awkward rear plugs
A torque wrench covering roughly 10 to 40 Nm
A wire or coin style gap gauge
Compressed air or a small brush to clear the plug wells
Dielectric grease for the coil boots
The correct new plugs, one per cylinder

Anti seize compound is often recommended online. Many plug makers now advise against it on plated plugs because it changes how torque translates into clamping force. Follow the instructions on the plug box.

How to Replace Spark Plugs Yourself, Step by Step

Work on a cold engine. Aluminium cylinder heads expand when hot, and removing plugs from a warm head is how threads get damaged.

Disconnect the battery if your coil packs share a harness with sensors you might disturb. On many cars it is optional, but it costs nothing.
Remove the engine cover and note how the coil packs or plug leads are routed. Take a photo.
Clean around each coil with air or a brush before lifting it. Grit falling into an open plug hole is the most avoidable cause of trouble.
Work one cylinder at a time. Unplug the coil connector, undo the bolt, twist and pull the coil straight up. Never pull on the wire.
Remove the old plug anticlockwise. If it feels stiff, stop, add a little penetrating oil, wait ten minutes and try again with gentle back and forth movement.
Check the gap on the new plug against the specification. Most iridium plugs come pre gapped; do not lever the fine centre electrode.
Start the new plug by hand. Use the socket and extension without the ratchet, or a short length of rubber hose, and turn until it seats. If it will not turn freely, it is cross threaded.
Torque to specification. Typical values sit between 18 and 28 Nm for aluminium heads with gasket plugs. Our guide on using a torque wrench correctly explains the technique.
Refit the coil with a thin smear of dielectric grease inside the boot, then move to the next cylinder.

Reading the Old Plugs Before You Bin Them

Old plugs tell you how the engine has been running. Line them up in cylinder order and compare.

Appearance	Likely meaning
Light tan or grey deposits	Normal combustion, nothing to fix
Dry black soot	Running rich, short trips, or a sticking injector
Wet oily coating	Oil getting past rings or valve seals
White, blistered insulator	Running too hot or too lean
One plug different from the rest	A problem specific to that cylinder

A single odd plug is the most useful clue. It narrows a fault down to one injector, one coil or one cylinder, which saves time later.

Mistakes That Turn a Cheap Job Expensive

Cross threading is the big one. Repairing a stripped thread with an insert kit can cost more than the whole service you were trying to avoid, and on some engines the head has to come off. Starting every plug by hand prevents it almost entirely.

Over tightening comes second. Without a torque wrench, many people lean on the ratchet until it feels firm, which on an aluminium head can mean double the correct force. Under tightening is less common but lets the plug overheat.

The third is mixing up coil connectors on engines where the harness reaches more than one cylinder. Working one cylinder at a time avoids it.

Frequently Asked Questions

Can I replace just one spark plug?

You can, but plugs wear at a similar rate, so changing the full set keeps combustion even across cylinders. Replacing one only makes sense if it was damaged or fouled by a fault you have since fixed.

How long does a spark plug change take?

On an inline four cylinder with coils on top, 30 to 60 minutes for a first timer. V6 engines with rear plugs under the intake can take three hours or more.

Should I change the coil packs at the same time?

Not by default. Coils fail individually and are expensive. Change one only if it shows cracks, carbon tracking or tests as faulty.

The Practical Takeaway

Buy the plug your engine was designed for, work cold, clean before you open each hole, start every plug by hand and finish with a torque wrench. Do those five things and replacing spark plugs yourself is one of the safest, cheapest jobs on the car. For the next job on your list, see our other DIY repair guides.