I have a 99 civic dx (hatchback) nearly a brand new cat, and two new oxygen sensors. I had a problem where the bolts holding my cat to my exhaust manifold (custom exhaust layout) were loose so they had to be retightened. The cat is in good shape but now I have that code on. Not sure what it means, although I know it has to do with the cat. But like I said, both cat and o2 sensors are almost brand new. Less than 15000 k on them. Not sure what the problem could be. Any ideas???

 

Clear/reset the ECU after making sure everything is correctly installed and tightened, and see if it returns.

No idea if the code means the same thing, but my 00 GSR threw that code 2 months ago, and it hasn't come back after clearing the ECU. Sometimes, you just get random codes as things get old. If they regularly occur, then you know something is probably wrong.

 

is it a high flow cat? cheap high flow cats are not effective enough, and cause p0420 codes. the code means the cat is not efficient enough. it's caused when the pre-cat o2 and post-cat o2 read too similarly. a good cat with cause the post-cat o2 to read leaner and with much less variance than the pre-cat sensor

 

Have you checked the exhaust system (Exhaust manifold, Muffler and Exhaust pipes) for leakage; leak in the exhaust system rises the error code. Also check the Engine Coolant Temperature sensor. Improper working of the ECT sensor results in the Check Engine Light to lit up and the vehicle throws error code p0420.

 

Tunern00b, I will definitely try that. It might be an old cold from when my cat was loose. Jmarshall098, the whole exhaust system is good other than when the cat was loose.
Motoxxxman, how can I tell what kind of cat it is? I really have no idea I've owned the car 2 months and the guy before me put it in.
And I appreciate all the input. Surprised at how quickly responds were. New to the forum. Much obliged.