Spark plug: when and how to replace it

When to replace it

At the manufacturer's interval
If there are misfires or an uneven idle
If the electrode is badly worn or dirty

Signs of wear

Jerking when accelerating
Hard starting
Engine warning light
Higher fuel consumption

What you need

Spark plug socket
Extension and ratchet
Torque wrench
Compressed air or blower

General steps

These steps are for guidance. Always follow your car manual and its torque values.

Work with the engine cold and remove the coil or lead from each spark plug.
Clean around the well so that no dirt falls into the cylinder.
Loosen and remove the spark plug without forcing it.
Check the part number and, if the manufacturer specifies it, the electrode gap.
Thread the new one in by hand to avoid damaging the thread and tighten to the specified torque.
Refit the coil and repeat, cylinder by cylinder.

PrecautionsDiesel engines do not have spark plugs but glow plugs, which are replaced differently. If a spark plug does not come out easily, stop: a damaged thread is expensive to fix.

Related OBD codes

Guide for your model

For instructions on your specific car, Autodoc Club publishes manuals by make and model.

Open the Autodoc Club manuals

External site, not related to OBDpedia.

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional.