Spark plug replacement

How to recognise worn plugs

• The engine idles unevenly.
• The car jerks when accelerating, especially uphill or when overtaking, and does not pull as it used to.
• The engine is hard to start, especially in freezing weather.
• Fuel consumption rises for no other obvious reason.
• The engine warning light comes on, and the fault memory holds a misfire code.

The same symptoms can come from an ignition coil, a plug lead, an injector or an intake air leak. If the plugs were replaced only recently, the fault is probably elsewhere. So we first ask when the plugs were last replaced, and we read the fault codes if the warning light is on. If the cause is not the plugs, we find it with diagnostics.

When the plugs are replaced

The car's manufacturer sets the interval, and it depends on the engine and the type of plug. It is usually 30 000 to 120 000 km. The shortest interval is usually on older cars with nickel plugs. On an engine fitted with iridium or platinum plugs it is often 60 000 km or more. On a turbocharged petrol engine it is often shorter than on a naturally aspirated one, because the pressure in the cylinder is higher and the spark needs more voltage.

Many manufacturers set a time limit as well as the kilometres, usually 3 to 6 years, and the plugs are replaced at whichever comes first. The time limit matters for a car that is driven little. On short trips, especially in winter, the engine never warms up properly, and carbon builds up on the plugs. A plug left in place for a long time can also seize in the cylinder head.

In the service schedule the plug change is usually assigned to a particular scheduled service, and it is done with that service. If the service book has no entry for a plug change and the car has covered more than the manufacturer's interval, the plugs are worth replacing.

How the work is done

1. We look up the plug type and the tightening torque for your car's engine in the manufacturer's data. If the warning light is on, we read the fault codes before the work. We tell you the price before we start.
2. We let the engine cool. On most engines the cylinder head is aluminium, and a plug removed from a hot head can damage the threads.
3. We remove the engine cover and the ignition coils or plug leads. We blow the plug wells clean with compressed air so that dirt does not fall into the cylinder.
4. We remove the old plugs and look at their tips. An oily, sooty or pale burnt tip tells us about the state of the engine. At the same time we check whether there is oil in the plug wells.
5. We start the new plugs by hand first, so that the thread does not cross, and tighten them to the manufacturer's torque. We check the gap on the plugs where it is adjusted. Iridium and platinum plugs usually come pre-gapped.
6. We refit the ignition coils or plug leads and check the rubber boots and connectors. Finally we clear the fault memory, road test the car and record the change in the service book.

How long it takes

On an ordinary four-cylinder engine the change usually takes from half an hour to an hour, and on a three-cylinder it is often quicker. On a transversely mounted V6 the plugs of the rear bank can often be reached only with the intake manifold removed, and then the work takes considerably longer. When we agree the price, we tell you when the car will be ready. The plugs can often be replaced at the same visit as a scheduled service or an oil change.

What the price depends on

The price of a spark plug replacement varies from car to car. It depends on:

• The number of cylinders and plugs. A three-cylinder engine has three plugs and a V8 eight. If each cylinder has two plugs, the number doubles.
• Access to the plugs. If the plugs can be reached only after the intake manifold or other parts come off, there is more work, and the manifold gaskets are replaced with new ones.
• The type of plug. Iridium and platinum plugs cost more than nickel plugs. We use the type the manufacturer specifies for the engine.
• Seized plugs. A plug left in place for a long time can be stuck in an aluminium head. We remove it carefully, but if the thread is damaged, repairing it is a different job.
• Extra findings. A cracked coil boot or oil in a plug well means that another part needs replacing too. We call you and tell you the price before we do it.

So we check the car and tell you the price before the work starts.

Spark plugs and the inspection

The plugs are not examined as such at the inspection, but a misfire shows up in the emissions check. Under Traficom's inspection regulation, a petrol car's emissions are checked by reading the fault memory of the car's OBD system. On a car more than ten years old, the exhaust is also measured at raised revs.

A fault reported by the engine warning light is grounds for rejection, and so is a code between P0001 and P0499 stored in the fault memory. The misfire codes P0300 to P0308 fall within that range. When the mixture in a cylinder does not ignite, unburnt fuel raises the hydrocarbon content of the exhaust, and the measurement can exceed the limit.

In Traficom's inspection statistics for 2025, the petrol engine exhaust measurement accounted for 31 338 rejecting faults and the OBD system for 30 779. The statistics do not say how many of the faults were caused by the ignition, because the same groups also cover lambda sensors, catalytic converters and other faults in the emission system.

Simply clearing the fault codes before the inspection does not fix the fault. If the misfire continues, the engine control unit stores the code again. The inspector also checks that the system's own readiness tests have been completed, and clearing the fault memory resets them. Because we clear the fault memory after replacing the plugs, drive the car normally for a few days before the inspection so that the readiness tests have time to complete.

If the car has already failed on emissions, bring the inspection certificate with you, and we see straight away what the inspector recorded. Read more about inspection repairs.

What you can check yourself

• Look in the service book or the electronic service history to see when the plugs were last replaced and what interval is set for the car.
• Notice when the symptom appears: with the engine cold or warm, at idle or when accelerating, in dry or wet weather. If the engine jerks mainly in damp weather, the cause is often an ignition coil or a plug lead.
• See whether the engine warning light is on steadily or flashing, and tell us when you book a time.
• Keep an eye on fuel consumption. If it has risen noticeably for no other reason, mention that too.

When not to drive the car

An engine that runs only slightly unevenly can usually be driven to the workshop, but a time is worth booking soon. Do not keep driving if

• the engine warning light is flashing
• the engine shakes heavily or the car cannot keep up with traffic
• there is a smell of petrol or burning from the car
• the engine cuts out while driving.

A flashing warning light usually means a misfire so heavy that unburnt fuel burns only in the catalytic converter and overheats it. The converter can be damaged after only a short drive, and replacing it costs several times as much as replacing the plugs. In that case, bring the car to the workshop on a tow truck. Call us and we will agree a time.