Spark plugs are small, inexpensive and easy to forget, yet they have a big effect on how a petrol engine starts, runs and uses fuel. Shelves are full of plugs that claim to fit dozens of cars. This guide explains what makes one plug different from another, and why the plug your car's maker specifies is the one to fit.
What a spark plug does
In a petrol engine, the spark plug creates the spark that ignites the air and fuel mixture in each cylinder. It does this thousands of times a minute, at very high voltage and in extreme heat and pressure. Diesel engines don't use spark plugs. They rely on compression to ignite the fuel and use glow plugs to help with cold starts.
Signs your spark plugs need replacing
- Harder starting, especially in cold weather
- Rough idle or a slight shake at standstill
- Misfires, hesitation or jerking under acceleration
- Higher fuel consumption
- An engine management light, often with misfire codes
These symptoms can also come from ignition coils, injectors or air leaks, so a proper diagnosis is worthwhile. Our guide to common engine fault codes explains some of the codes you might see.
Heat range: why it matters
Every plug has a heat range, which describes how quickly it moves heat away from its tip into the cylinder head. A plug needs to run hot enough to burn off carbon deposits, but cool enough to avoid glowing and igniting the mixture too early.
- A plug that runs too cold can foul with carbon and misfire.
- A plug that runs too hot can cause pre-ignition or knock, which can seriously damage pistons, particularly in modern turbocharged engines.
Engine makers choose the heat range carefully for each engine. Two plugs that fit the same thread can have very different heat ranges.
Copper, platinum and iridium
The material at the electrode tip affects how long the plug lasts and how well it sparks.
- Copper-core plugs with nickel electrodes are inexpensive but wear relatively quickly. They are mainly found on older engines.
- Platinum plugs resist wear better and last longer.
- Iridium plugs are very hard and allow a very fine centre electrode, which needs less voltage to spark. Many modern engines specify iridium, often with long change intervals.
Fitting a cheaper material than the engine was designed for can mean shorter plug life and poorer running. Fitting a more expensive one doesn't necessarily help if the heat range or design is wrong.
Gap, reach and design
Other details matter too:
- Gap: the distance between the electrodes. Many modern plugs are pre-gapped and shouldn't be adjusted, as fine iridium tips can be damaged.
- Reach: the length of the threaded section. Too long and it can hit the piston. Too short and it may not sit properly in the combustion chamber.
- Seat type and thread size: these must match the cylinder head exactly.
- Electrode design: some engines need plugs with several ground electrodes or a specific tip shape.
Why the factory specification is the one to fit
When an engine is developed, the manufacturer tests plugs for heat range, durability, emissions and performance. The plug it lists in the parts catalogue is the result of that testing. Volkswagen, Toyota, Hyundai and other makers each specify plugs by part number for each engine, and sometimes change that specification during production.
Choosing the genuine part number for your exact engine means you get the same plug the engine was signed off with. Our guide to reading a VW Group part number explains how to spot updated versions, and our guide to using your VIN shows how to confirm your engine.
How often to change them
Intervals vary a lot. Older engines with copper plugs might need new ones every couple of years, while many modern engines with iridium plugs run for several years or tens of thousands of miles. Turbocharged engines are often on the shorter end. Check your service schedule, which may give both a time and a mileage limit.
Fitting tips
- Let the engine cool before removing plugs, especially on aluminium cylinder heads.
- Tighten to the manufacturer's torque figure. Overtightening can damage threads; undertightening can let the plug overheat.
- Check the ignition coils and their boots for cracks while they are off.
- Change all the plugs at once, so every cylinder runs evenly.
Find genuine spark plugs
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