I have a 99 civic ex. It has 130k miles on it. Going down the road today the Check engine light come on. Had it scanned at autozone and they came up with error P0420. Guy said my CAT Conv. was bad. Guy i know is a mechanic he said more than likely its just a bad sensor? Where should i start as i hate just throwin parts at it? Thanks

 

Check for an exhaust leak between the primary and secondary O2 sensors. If you don't find a leak, then the cat is likely bad. To verify, remove the cat and inspect it. The catalyst should have a honeycomb structure. If it looks melted, replace it.

 

i had the same problem with my D16Y7 LX. so first i diagnosed the cat to see if it was really bad. here are some few signs that your cat is bad.
1. it smells like rotten eggs from your exhaust.
2. if you tap the cat with a rubber mallet and you hear stuff rustling inside then most likely it's going bad.
3. hold your hand close to the exhaust pipe when you're idling. if it has irregular pulses of exhaust then it's probably clogged.

but most of the time it's the O2 sensor after the catalytic converter. i changed mine and it hasn't turned on ever since. just remember to erase the code not from the OBD-2 reader but from your car itself. when the Check Engine Light come on. your car has a back up memory in which it stores the trouble code and usually the only way to fix it is first fix the problem then clear the code. you can do this by pulling out the BACK UP fuse from your fuse box under the hood. pull it out wait at least then seconds then put it back in. it should clear it.

if you don't do this the CEL will come on again even after you fixed the problem.

 

I have a question. I just installed a new manifold converter. To clear the code in the computer can I just take the battery wires off or do I have to pull the fuse also?

 

Just remove the 7.5A BACKUP fuse for a minute or so and you should be good to go.

 

Cool beans. Now to see if the oxygen sensors, which I did not replace, throw a code(s). Since I installed the manifold converter the car sometimes chokes off at stop lights and stop signs..but not always.Sometimes it purrs like a kitten. That happened years ago,but I forget what the cause was now. I will wait to see what code gets thrown.

 

I pulled the fuse and it must have cleared the code cause my car passed inspection. WooHoo!!
It also stopped choking off at stoplights...or at least it has not done it recently.
Next on my TO DO list is to replace the fuel filter and replace the PCV valve. I also want to clean up the Idle Air Control Valve...if i can find it.