Spark Plug Replacement: Signs & Costs
Small but mighty, spark plugs are the heartbeat of your petrol engine. Here is when and why you should replace them.
What do spark plugs do?
In a petrol engine, spark plugs provide the electrical ignition needed to combust the fuel-air mixture. Over time, the electrodes on the plug wear down, increasing the "gap" and making it harder for the spark to jump. This leads to inefficient combustion, poor performance, and eventually, engine misfires.
Note: Diesel engines do not have spark plugs; they use glow plugs to pre-heat the cylinders instead.
5 Signs Your Spark Plugs Need Replacing
Difficulty Starting
If your car struggles to turn over or takes multiple attempts to start on a cold morning, worn plugs may not be providing enough spark.
Engine Misfiring
A "stuttering" or "jerking" sensation while driving, often accompanied by the check engine light, is a classic sign of a plug failing to fire.
Poor Fuel Economy
Incomplete combustion means you are wasting fuel. Worn plugs can reduce your MPG by up to 30%.
Rough Idling
If the car vibrates or sounds "clattery" when stopped at traffic lights, the plugs may be struggling to maintain a consistent rhythm.
Slow Acceleration
When you put your foot down and the car feels "sluggish" or delayed, it"s often because the spark isn"t strong enough for high-demand situations.
Spark Plug Replacement Costs (UK 2024/25)
The cost varies based on the number of cylinders in your engine and the type of plug (Nickel, Platinum, or Iridium).
Estimated Costs
- Parts only (Set of 4): £20 – £60
- Labour (Standard Garage): £40 – £80
- Total Fitted (4-cylinder): £60 – £140
- Total Fitted (6-cylinder): £100 – £200
Will worn spark plugs fail an MOT?
Technically, spark plugs are not a specific test item in the MOT. However, worn plugs cause high emissions and can trigger the "Engine Management Light" (EML). Both of these are Major Failures. Ensuring your plugs are healthy is a vital part of your MOT preparation.
Frequently Asked Questions
How often should I change them?
Standard plugs usually last 30,000 miles. Long-life Iridium or Platinum plugs can last up to 60,000 or even 100,000 miles. Check your manufacturer's service schedule.
Can I change them myself?
On many older cars, it's a simple 20-minute job with a spark plug socket. However, on some modern cars, the plugs are hidden under the intake manifold, requiring significant disassembly.
Keep Your Engine Healthy
Regular maintenance prevents expensive repairs. Check your car's full service and MOT history to see when yours were last replaced.
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