Always thought the 'check engine' light was a simple sensor thing, but my scanner showed something else

I was working on a 2017 Civic with a steady check engine light. My old scanner just gave a generic P0420 code for the catalytic converter. Then I borrowed a buddy's high-end scanner from his shop in Phoenix. It showed the downstream O2 sensor voltage was stuck high, not just low like a bad cat. Found a tech bulletin online saying a faulty thermostat can cause that by keeping the engine too cool. Replaced the $40 thermostat first, cleared the code, and it's been good for two weeks now. Anyone else run into a weird fix for a P0420 that wasn't the cat?

2 comments

2 Comments

the_avery1d ago

Heard of a bad spark plug causing a P0420 on some older Toyotas.

janah381d agoMost Upvoted

That's actually a P0300 series misfire code, not a P0420. The cat code usually comes after the misfire ruins it lol.