Ignition
Change Spark Plugs
Take out the old spark plugs one at a time on a cold petrol engine and fit new ones by hand, then to the handbook figure.
- MediumNeeds more tools and care, and often a safely raised car.
- 30 to 90 minutes
- Updated
General guide; check your car's handbook.
Safety first
- Work only on a cold engine. A plug thread in a warm aluminium head is easily damaged.
- Switch off the engine and take out the key before you touch any ignition part. The ignition makes very high voltage.
- Change one plug at a time, so the leads or coils go back on the right cylinders.
- Never force a plug that will not turn by hand. A cross-threaded plug can ruin the cylinder head.
Steps
- Get the right plugs. Plugs differ by engine, so choose them for your car's engine and model years ("Parts for your car" below), and buy one for every cylinder. Your handbook says how often they are changed.
- Let the engine cool completely, for some hours. Open the bonnet and take off any plastic engine cover.
- Clean around the first plug. Blow or brush away dirt and sand around the coil or lead, so nothing falls into the cylinder when the plug comes out.
- Take off the coil or lead from the first plug. Unplug a coil's connector, undo its bolt and pull it straight up with a gentle twist. Pull a plug lead by its boot, never by the wire.
- Undo the old plug with the spark plug socket and the extension, turning it anticlockwise, and lift it out.
- Look at the old plug. A light brown tip is normal. Black soot, wet oil or white deposits can point to an engine fault: tell your workshop at the next service.
- Check the gap only if the box says so. Many modern plugs (iridium or platinum) are set at the factory and must not be bent. If yours may be gapped, set it with the feeler gauge to the figure on the box or in the handbook.
- Start the new plug by hand. Put it in the socket and turn it in by hand for several turns. It must turn easily; if it does not, take it out and start again straight.
- Tighten the plug to the figure in your handbook or on the plug's box, with the torque wrench. Too tight can crack the plug or strip the thread; too loose lets it overheat.
- Refit the coil or lead firmly until it clicks or seats, then its bolt and connector.
- Do the next plug the same way, one at a time, until all are done. Refit the engine cover.
- Start the engine. It should idle smoothly with no engine light. Write down the date and the mileage.
When to go to a workshop
Go to a workshop if a plug will not come out or turns stiffly all the way, if a thread is damaged, if the plugs sit deep under the intake and you cannot reach them, or if the engine still misfires or the engine light stays on after the change. A broken plug or a damaged thread needs special tools.
Parts for your car
Choose your car and we open its Spark Plugs page.