My dad has a 2000 Civic LX manual (same as mine!) and is having a strange issue with a P0420 code. He drives about 5 miles to get on the interstate, gets on the interstate and sets the cruise to 65ish. After about 2 miles, almost at the same spot on the road the CEL comes on with a P0420 code. He has a code reader attached and shuts the code off immediately and it doesn't come back on for the rest of his 30 mile trip, or even further if he drives further.


If he goes 70mph the light doesn't come on. He replaced the 02 sensors but it came on again at the same place on the highway. Seems to happen more frequently when it is cold than hot out. Sometimes does it on his way back home but not as often.


Doesn't seem to happen on city driving.

Has anyone else experienced this? I didn't think it was a cat since it doesn't come back on right away.


Thanks

The code won't come back on right away because the sensors need time to reset. There are some codes that pop back on immediately (like the ones that monitor the engine directly: CKP sensor, TDC sensor, etc.), but things like O2 sensors don't come back right away. The 420 code definitely means your cat is on it's way out.