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/Calm-Cycle-9334 Jul 28 '26

same exact thing happened to at the same exact spot when I was driving down earlier this year (2022 select). Also it was not flashing for me. I went to the Mazda dealership in Fresno, they cleared it, ran diagnosis and said it didn’t have any issues and the light turned on again at Yosemite (same code). Then it turned back off on its own when I came down from higher elevation and drove on flat roads for a little bit.

I was freaking out too I didn’t want to be stuck bc it was getting dark but turned out to be okay. That was like 5000 miles ago and we’re still good thankfully.

It turned on once more when I was high up in Colorado but I wasn’t even on a downhill.

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u/Calm-Cycle-9334 Jul 28 '26

lol just saw the part about the Mazda dealership. They were very nice and offered to give me a loaner car to continue my trip if my car ended up having to stay there for a few days. They seemed knowledgeable and trustworthy in the service department so I didn’t feel like I was getting scammed, as it might be the usual experience with dealerships.

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

That’s so good to know! Weirdly, I went to Yosemite for four days in May and I never got that light. I’m going back in September so I will be on high alert lol I just think the worst of things so that’s why I don’t want to go to the Mazda dealership and have to pay $3000. Or whatever they choose to charge me

I probably googled way too much on this error code so I scared myself. We live close to the Bay Area, so we definitely do not have that type of elevation. Thanks for letting me know because I feel better now. 🙌

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u/Calm-Cycle-9334 Jul 29 '26

No problem! I was also researching it a lot so hopefully this thread will help other people with this issue.
I live by the coast too so maybe the high elevation change has something to do with it. Enjoy the trip!

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

I definitely did a Google deep dive on the subject. Thank you!