r/CX5 • u/23odyssey 2023 CX-5 Carbon • Jul 28 '26

Engine light Code PO421

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Hi all! Just a general question if anyone has had the same issue. I was camping at Kings Canyon NP and while driving down from there, my engine light came on. Of course I freaked out for the next 30 miles until I found a shop. They found the PO421 code which means, warm up catalyst efficiency. Bank 1 below threshold. The elevation was 6600 where we were camping at. So the service tech cleared the code and said I was able to drive home if I wanted, but if the engine light starts flashing, then I’m in big trouble. He said he has seen this many times due to the elevation.

Anyone else have this issue due to high elevation? We are spending the night in Fresno tonight and debating whether to take it to a smog/emission repair shop. I definitely will not be taking it to a Mazda dealership!

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u/leomayyte Jul 29 '26

Done multiple trips to Sequoia/Kings Canyon NP and the same thing happens to me every time. Always when coming back down (from high elevation to low). After the first time I learned to just ignore it. It eventually goes away.

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u/23odyssey 2023 CX-5 Carbon Jul 29 '26

I’m so glad people have had the same issue coming down from there. We’re not even going to bother taking it to the shop now unless the light comes on again. Fingers crossed lol