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Hey all, I'm chasing down a problem on my daughters 2000 civic.
We've had it for a year and a half or so. Initially I replaced the exhaust manifold & catalytic converter & o2 sensor cause the manifold was cracked and it was throwing a P0420 code. It also didn't want to start in the cold.
I ended up putting new plugs & wires in it also. Well... it's cold in MN again and it's doing the same thing.

All of my research for P0420 says to replace the cat and/or o2 sensor which I did just over a year ago. What thoughts do any of you have???

 
Rep Power: 367

you can get by using a defouler for now.

if fuel is too rich for some reason, it could clog the cat pretty fast. Or oil. but those are guesses

 
Rep Power: 222

Did you buy a cheap aftermarket converter? The computer is pretty picky

 
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I bought one from RockAuto.

Just bringing this thread back to life now that it's cold again here in MN. The car ran fine all summer but still had the P0420 code. We ignored it as it was warm out.
Now that it's cold again the car wouldnt' start in the cold. The battery is fully charged. I pulled the plugs and they all are burning pretty good, but were covered in gas. My guess is that this just happens on cold startup though since that is the only time it has a hard time starting.

Any ideas of where to check next & how to check it?

 
Rep Power: 222

They sell good ones and not good ones, which one did you buy? Also what brand was the sensor?

 
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Apparently it was Amazon, not Rock Auto.
ATP Automotive exhaust manifold & catalytic converter.
Bosche O2 sensor.
After testing a bunch of things today, it looks like my cat is bad. The primary O2 sensor stayed within 0-1V as I drove it and the secondary O2 stayed as stable as possible around 0.5-0.7V.
Checked the temp on the cat and it's around 600 on the inlet & 300 at the outlet.

I don't know if this will fix my cold starting problem, or if this was a result of the cold starting problem? The car runs great otherwise.

 
Rep Power: 222

Bosch is the wrong sensor needs to be Denso/ntk.
wrong/bad primary o2 can cause all sorts of running/starting issues.
Cat is probably cheap garbage (if you didn't spend like $800 it's not going to be good enough for the computer). You can put a defouler on the secondary only and it will get rid of the p0420.