Spark plugs are a routine replacement item on every petrol engine. They ignite the air-fuel mixture in the combustion chamber, and over time the electrode wears down, the gap widens, and the spark becomes weaker. Replacing them restores engine performance, improves fuel economy, and ensures smooth idle. This guide covers the full procedure.
When to Replace Spark Plugs
Replacement intervals depend on the plug type. Standard copper-core plugs typically last 30,000 to 60,000 km. Platinum plugs last 60,000 to 100,000 km. Iridium plugs can last 100,000 to 160,000 km. Your workshop manual lists the exact interval and the correct plug part number for your engine. Do not assume all engines in the same model use the same plug – different engine variants often require different heat ranges and gap specifications.
Signs Your Spark Plugs Need Replacing
Rough or uneven idle, hesitation or misfires during acceleration, reduced fuel economy, difficulty starting (especially in cold weather), and a check engine light with misfire codes (P0300 to P0308) are all common symptoms of worn spark plugs. See our OBD-II codes guide for more on misfire codes.
Tools Required
You will need a spark plug socket (a deep socket with a rubber insert that grips the plug – typically 16 mm or 21 mm depending on the engine), a ratchet with extension bars, a torque wrench, a gap gauge or feeler gauge, and anti-seize compound (for aluminium cylinder heads). Some engines with coil-on-plug ignition also require removing the ignition coils, which may need a specific Torx or hex bit.
Step-by-Step Replacement
Work on a cold engine to avoid burns and to prevent cross-threading plugs into hot aluminium. Remove one plug at a time to avoid mixing up ignition leads or coil connections.
Disconnect the ignition coil connector and unbolt the coil (on coil-on-plug systems) or remove the spark plug lead by pulling on the boot, not the wire. Use compressed air or a brush to clean any debris from around the plug hole before removal – you do not want dirt falling into the cylinder.
Unscrew the old plug using the spark plug socket. If the plug is seized, apply penetrating oil around the base and let it soak for 15 minutes before trying again. Never force a seized plug, as snapping one off in the cylinder head is a much bigger problem.
Check the electrode gap on the new plug with a gap gauge. Most modern engines specify a gap between 0.8 mm and 1.1 mm, but this varies – your workshop manual has the exact specification. Iridium and platinum plugs are often pre-gapped from the factory, but it is worth verifying.
Thread the new plug in by hand first to avoid cross-threading. Use the spark plug socket and extension to start it, turning by hand until you feel it seat against the cylinder head. Then torque to specification. Most plugs in aluminium heads require 15 to 25 Nm – over-torquing can crack the threads in the head, and under-torquing can allow the plug to blow out under compression. Your manual lists the exact figure.
Reconnect the coil or plug lead and move to the next cylinder. Once all plugs are replaced, start the engine and let it idle for a minute to confirm smooth running.
Reading Old Spark Plugs
The condition of your old plugs tells you about engine health. A light tan or grey electrode is normal and indicates correct combustion. Black sooty deposits suggest a rich fuel mixture or oil burning. White or blistered electrodes indicate overheating or a lean mixture. Oil-wet plugs suggest worn valve seals or piston rings allowing oil into the combustion chamber. Your workshop manual includes a spark plug condition chart with colour photographs for comparison.
Get the Right Specification
The correct plug type, gap, and torque value are engine-specific. Your workshop manual provides all of these for your exact vehicle. MechanicMate offers PDF workshop manuals for over 960 models at mechanicmate.net/shop.
