Hi there. I have a 2002 Honda Civic LX with 260,000 miles. I keep getting the P0420 code for the Catalytic system bank 1 (exhaust manifold). I cannot figure out what’s wrong for the life of me.

I’ve replaced the catalytic converter, O2 sensors, PCV valve, spark plugs, air filter, gaskets, and just cannot get it to go away!!!

The car runs perfectly fine which is why I have 0 clue on what it could be. I spoke to my uncle and he said that it might actually be a bad alternator not providing enough juice to the electrical system or even too much. I got it tested and it seems fine.

Any tips?

 

where did you purchase all the parts that were replaced ?

 

Everything was from Autozone except the catalytic. I ordered it online. Same store from another cat I bought years ago and never had issues. I thought it was that, got it tested and it’s doing its job

 

Am assuming you have cleared the codes and it came back??
Then, would suspect you bought an after market O2 sensor??
Honda vehicles are sensitive to aftermarket electronic parts
If you bought Denso O2 sensor and it is the right O2 sensor (downstream sensor)
The you will need a scanner and watch the O2 sensor output on graph and see what it is doing
The downstream O2 sensor should produce a fairly steady state voltage

Note: Ordering on line doesn't mean you always get the real parts
Lots of fakes out there. Hard to tell.

 

I bought Bosch sensors, the voltage on them fluctuates between .3 &.7 volts which seems normal. They’ve been on since November without issue. As for the catalytic, it was pretty cheap so that might of been it. It seems to be working fine though and bank 1 is giving me issues, not bank 2

 

Bosch on Honda vehicles are known to produce issues such as this.
It is a hit or miss type of thing.
But, you may be right. That the new cat just isn't performing as good as it should.

 

hmm. i’ll switch over to denso O2 sensors, but first I’m gonna use one if those catalytic cleaners. Dont care if I ruin the Bosch sensors lol

i’ll let you know if it fixed it by this weekend. I’ll do an Italian tune up too lmao

 

dont use the aftermarket sensor. Honda has theirs made slightly different than aftermarket. same as the trans fluid. (just some info)