99 Civic EX - P0420 Code
 

Hi All,

I just replaced my upstream and downstream O2 sensors and now I'm getting a P0420 code. I had my catalytic converter replaced a little more than a year ago, so I'm hesitant to have it replaced again. The car does have some issues when cold. When accelerating from a dead stop, the car stumbles (has new plugs and wires) and doesn't accelerate like it should. I think the P0420 code may be somehow related to the stumbling issue. Anybody experienced something along these lines before? Any suggestions on what to look at first?

Thanks

Re: 99 Civic EX - P0420 Code
 

A rich running engine can kill a cat pretty quickly. It's best to fix O2 sensor issues first and then replace the cat.

If the cat turns out to be fine, then look for an exhaust leak.

Re: 99 Civic EX - P0420 Code
 

I just replaced both O2 sensors about a week ago. Now I have the CEL and P0420 code. I'll check for exhaust leaks.

Re: 99 Civic EX - P0420 Code
 

do u have an exhaust on your honda? because ever since i got my exhaust i can't get that damn code to go away and i know my cat and o2 sensors are good

Re: 99 Civic EX - P0420 Code
 

Nope, the exhaust is an OEM replacement. My car is overdue on the inspection, so I'm trying to get things in order so it'll pass.

Re: 99 Civic EX - P0420 Code
 

So I pulled the backup (7.5A) fuse last night to reset the computer. After just about 10 miles the CEL is back on. Now it seems to hesitate and stumble a bit more. Could the new O2 sensors be bad? I'm going to grab the CEL code tonight when I get home.

Re: 99 Civic EX - P0420 Code
 

Anybody? Could a bad coil result in enough unburned fuel to trigger a P0420?

Re: 99 Civic EX - P0420 Code
 

Hello everybody. I have the same problem (1998 LX) and I think it's most likely the cat as the ECU indicates. Through some research on the net I found this TSB which might aid you as you mention the coil. It involves crimping the connectors at the igniter terminals.

http://www.tegger.com/hondafaq/p0420/A05-027.pdf

If it was the sensors it the ECU would throw more codes. I have an aftermarket 2 piece (flanged) exhaust manifold / catalytic converter and I myself am wondering if this two piece is better than a one piece unit. I've also heard of a bad ECT (engine coolant temperature) sensor that could throw this code but again wouldn't this throw a different code?

Re: 99 Civic EX - P0420 Code
 

my check engine light came on days ago.. Checked code today.. 420.. As I was on the way to pick up my girl my car started over heating.. I'm now doing research and stumbled upon this.. I'm going to try and replace the ect first to see if this fixes my issues

Re: 99 Civic EX - P0420 Code
 

check if the cat glows red

Re: 99 Civic EX - P0420 Code
 

Bad ECT sensor would not cause overheating. You need to troubleshoot the cooling system.

Re: 99 Civic EX - P0420 Code
 

oh no.. I know I have need of a new radiator.. It has a very small leak.. Just not ready to drop money.. Shiny wanna go cheap and can't do anything else at the moment

Re: 99 Civic EX - P0420 Code
 

I have this P0420 code as well, minus the overheating thankfully. I noticed somewhere along the way that alldata or something said to address all other codes before a replacing the cat. The cat is made to last the life of the vehicle or at least 10 years often warrantied 8 years or 80k miles. So after you figure out the other issues replacing the cat should be under warranty and the labor should be covered too since standard shop warranty can reasonably be expected to cover more than one year. So if it doesn't cost too much it might be worth the change but you don't want to foul up the cat with whatever the other codes\problems are.

I have an engine transplant from 2000 the D16A in my 04 civic ex. So a JDM 1.6L Sohc replacing a 1.7L. So I'm suspecting that the ECU needs to be adjusted for the difference.