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In the old days, a tune-up was necessary about every 35,000 miles. It would usually consist of setting the ignition timing, replacing the mechanical breaker points in the ignition, cleaning and adjusting the carburetor and replacing the plug wires and spark plugs. Today, of course, the carburetor’s job is done by fuel injection and the ignition timing and spark are controlled by the engine computer. Few vehicles still have plug wires anymore either, as the distributor was replaced by the computer and a coil-on-plug design which delivers a spark at each spark plug.

But what about the spark plugs themselves, though? How often do they need to be replaced now?

Manufacturers tout an 80k-100k mile service interval on spark plugs now, thanks in part to improvements in plug design and materials. That might be stretching it, however. Remember that if you have a 100,000-mile spark plug, its electrode is worn down 4/5 of the way at 80,000 miles. A worn electrode means a wider spark plug gap, which can mean a loss of power and fuel economy.

Worse, though, spark plugs that have been in the engine’s cylinder head for years and tens of thousands of miles have a tendency for their threads to seize. A seized spark plug can be pretty difficult to extract from the head and can mean a hefty repair bill before it’s all said and done.

If your vehicle was originally equipped with a specific type of spark plug, it’s a smart idea to keep that design of plug when you replace them. It’s also a good idea to check a few other things under the hood when it’s time to replace the plugs, including

• Air filter
• Cabin air filter
• Belts and hoses
• Charging system
• Starter
• O2 sensor
• All vacuum lines and junctions

Today’s cars may be a lot less maintenance-intensive, but that doesn’t mean “maintenance free.” Remember that a well-maintained car is a reliable and strong-running car…and don’t put off maintenance like spark plugs just because it doesn’t need to be done very often!