got2run5

Just got back from college, and the civic suffered a long winter. I barely drove it this year compared to normal so it sat sometimes for a couple weeks. Anyway, I got a CEL for my upstream 02 censor and while waiting for the school year to end to do some work, the car's performance went down the tubes. I admit, I saw "02 slow response" and I just cleared the code until I could find time to work on it since it seemed to be running lean not rich. After startup, accleration would be jerky and power way down. Yesterday I replaced the 02 and acceleration greatly improved and the car ran noticeably smoother but after about 30 miles I got CEL P0420--Cat below efficiency threshold. I've done some reading on the subject and my understanding is that cats can fail for 2 reasons-being clogged or not cleaning the exhaust enough. Since getting the CEL means that the upstream and downstream 02 censors are too similiar, I would think that if you got the CEL that would mean that your cat is NOT clogged but rather isn't cleaning the exhaust enough. Is this correct logic? I'm already spending enough cash on other parts of the car and I'd prefer not to buy a new catalytic converter as well. If it's clogged, I'd be forced to spend the cash but if it was just not completely effective I was thinking of going the route of an 02 defouler since WI emissions is kind of a joke.

ezone

P0420 is for cat inefficiency.
Your theory of the cat "cleaning" the exhaust is pretty close, the code has a lot to do with the cat losing its ability to store and release oxygen.
Causes of failures may vary, but misfires and excessively rich running can definitely kill a cat, in a matter of only minutes in extreme cases.


If you can floor the gas pedal and have the engine pull strong all the way up to redline, then the cat is not (significantly) clogged. If it runs out of "oomph" before redline, then you may have significant exhaust restriction.

got2run5

Thanks ezone, I was looking for some expertise such as yours. It's a relief to only spend $6 for a "patch" until the cat does start majorly restricting flow.