My 2017 Civic’s check engine light came on yesterday, and the code reader says it’s P0420 for catalyst system efficiency below threshold. The car seems to run fine, no odd noises or loss of power, but I’m worried this is the start of a costly repair. Has anyone had this code without any other symptoms, and what was the actual cause in your case?
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What usually causes a P0420 code with no other symptoms in a Civic?
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NoraA
Junior Member
AbigailQR
Junior Member
I had a P0420 on my Civic a while back. No obvious symptoms, ran fine. What helped was tracing the sensors first: the upstream O2 sensor was worn, I replaced it and the light stayed off afterward.
Grace.G
Junior Member
For me the issue showed up with no driveability problems, and it ended up being a flaky downstream sensor reading. Replacing that sensor fixed it and the cat seemed fine after that.
Gary.H
Junior Member
It did feel overhyped at first, like a ghost code. I kept wondering if there was a small leak or just a sensor hiccup rather than a dead converter.
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