When should I change spark plugs?

Copper plugs: every 20,000 miles. Platinum: 40,000–60,000. Iridium: 60,000–100,000. Always check your handbook for your exact engine.

Here's a quick, honest answer from a working UK mechanic — what causes it, what it costs to fix, and what to do next.

Will old plugs cause MOT failure? — Indirectly — misfires raise emissions which fail the test.

What about diesel engines? — Diesels have glow plugs, not spark plugs. Glow plug change is on a separate schedule, typically 60,000+ miles.

Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email bob@bobsmechanicalrepairs.co.uk.