Spark Plug Replacement Interval: When to Change Them and What Happens If You Don't
When your car sputters, idles rough, or refuses to start on a cold morning, the culprit might be something small but critical: your spark plugs, small but essential components that ignite the fuel-air mixture in your engine’s cylinders. Also known as ignition plugs, they’re the heartbeat of your engine’s combustion process. Most manufacturers recommend replacing them every 30,000 to 100,000 miles, but that’s just a starting point. Real-world driving—stop-and-go traffic, short trips, poor fuel quality—can wear them out much faster.
Ignoring a worn spark plug doesn’t just mean slower acceleration. It directly hurts your fuel economy, how efficiently your car turns fuel into motion. A misfiring plug forces the engine to burn more fuel to make the same power. It also increases emissions, which can cause your car to fail its MOT. Over time, bad plugs can damage your catalytic converter—a repair that costs hundreds more than a simple plug change. And if you keep driving with failing plugs, you risk engine performance, the engine’s ability to deliver power smoothly and consistently slipping so far that your car starts shaking or stalls unexpectedly.
There’s no magic number that fits every car. A 2010 Ford Focus might need new plugs at 40,000 miles. A newer Toyota with iridium plugs could go past 80,000. But here’s what you should watch for: rough idle, trouble starting, poor acceleration, or a check engine light with a misfire code. If you’ve hit 60,000 miles and haven’t checked them, it’s time. You don’t need to be a mechanic to spot the signs—many of the symptoms are loud and obvious.
The posts below give you real, no-fluff advice from UK drivers who’ve been there. You’ll find how to tell if your plugs are failing, whether changing them actually improves performance, and what happens if you wait too long. No theory. No marketing. Just what works on the road.
