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I have a 2000 Mazda Protege that last had it's spark plugs replaced about 4-6 years ago (about 40k-60k miles). It's a commuter car, but it's a short commute and mild driving conditions; probably 85% of its total miles are highway. When I replaced the plugs back then, I used Platinum (or similar) plugs, and replaced the wires at the same time. Currently I'm not having any noticeable problems (misfires, hesitations, poor MPG, etc). My questions are:

  • Given that the car is running fine, is it necessary, or at least still recommended, to change them by now?
  • Am I likely to see any improved performance if I do, or would it just be preventative maintenance?
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It is worth removing them, cleaning them and setting the gap if you don't want to replace them and they are still in good condition. Spark plugs will be a scheduled item to replace, like you said, for preventative maintenance. Unless the plugs were very dirty, worn or had a larger gap, you shouldn't notice any difference.

  • 2
    You can jam this chart into your answer so the OP can see if he should replace them, if you want. turbododgeparts.com/assets/images/plug-problem-matrix.jpg – DucatiKiller Mar 2 '16 at 19:32
  • Agree with this. The idea of scheduled replacement is to fix a worn part before it completely fails. A good inspection of the part can certainly extend the interval if everything looks good. – JPhi1618 Mar 2 '16 at 20:02
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As a tune up you can change the spark plugs in between 80-100, the ones that come stock are good ones so doesn't need replacement until they go bad or reach 80k, we at our shop replace them every 100k along with the timing chain or belt.

In your case the car will have a tiny I4 motor so the spark plugs are right on the top, the spark plugs would only cost about 20 bucks for this.

In your case you said the spark plugs were replaced around 50k, so after 5 years having 10000 miles per year you have put 50000 total. So I would say the spark plug should be fine for another couple years or 20000 miles, but if you have the time and 20 bucks go ahead replace them, it will only make it better, do this along with an oil change or something.

I don't think you will be able to see any major change in performance, because your spark plugs seems to be working fine now as there are no warning lights on the dash, and you have used some good spark plugs when you previously changed them.

Some tips: Don't over tighten the spark plugs, don't use any dropped spark plugs. screw them in using a 16 inch socket and and extension with hand all the way in and only turn about quarter to half a turn after they are tight with a ratchet.

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