Q: What is a PO421 code on my 2000 Mazda Miata? on 2000 Mazda Miata MX-5

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It only has 19000 miles on it. In addition, it needed an O2 sensor and a tuneup including a engine ignition coil.
(1) Answer
(1) Comments
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Mileage is very low 19,000 miles.Has this code been come on only since the coil and spark plugs were replaced. If the car is infrequently drive, put some good fuel injector, cleaner in the tank and go for a good hard long drive in the car and see if the code returns. If the check engine light continually flashes drive easy and return home. I just see your address I live in California used to go on holidays to Beach Haven/ Long Beach Island for many years when I was young.
Yes I do live near LBI. That's the reason I moved here after vacationing 20+ years on LBI.

Yes the engine light came on steady after the coil and spark plugs were replaced.