When to change spark plugs: signs and readings
Plugs die gradually and the car gets used to it. Here is what the numbers show before you feel anything.
Spark plugs are the cheapest consumable that affects everything at once: consumption, pull, smoothness and the life of the catalyst. And the least noticeable: they do not break, they weaken.
What the readings show
Misfire counters. The main sign. A tired plug misfires only under load at first — uphill, under acceleration — and at idle nothing shows at all. Look at the counters after a motorway run, not while parked.
Fuel trims. Unburnt fuel goes into the exhaust, the oxygen sensor sees the extra oxygen and the unit starts adding fuel. A steady positive trim with no air leak is a reason to look at the plugs.
Consumption. It creeps up by a litre or more with no other cause.
What your eyes show
Take one out and look at the electrode:
- Light brown deposit — that is what a healthy plug looks like.
- Dry black soot — a rich mixture: the air filter, the air meter, the injectors.
- Black oily deposit — oil in the chamber: valve stem seals or rings.
- A white electrode with melted spots — overheating: the wrong heat range or knock.
- A rounded electrode and a wide gap — simply worn out.
When to change them
Ordinary nickel plugs last 20,000–30,000 km, iridium and platinum ones 60,000–100,000. The exact interval is in the handbook and matters more than any general figure: turbocharged engines want them noticeably sooner.
What not to do
Fit «better» plugs than specified. The heat range is chosen for the engine, not by price: too «cold» a plug fouls up, too «hot» a one overheats the electrode and knocks.
Wait for them to fail. Misfires kill the catalyst, and it costs tens of times more than plugs.
