Hey fellas I'm currently getting a check engine light with code P0420. Catalyst efficiency below threshold Bank 1.

all the research I can come up with says that it's probably a clogged cat or a bad O2 sensor. I don't think it's the cat because I have plenty of power. Normally with bad cats or clogged cats the car is very low on power. I am not having that problem. The problem I am having is that my gas mileage decreased by half ever since that code appeared. A lot of things I've read online said that it could also be a bad O2 sensor on Bank 1. So my question is if it is a bad O2 sensor on bank 1 where is Bank One? Or where is the bad O2 sensor located if it's reading that code. The O2 sensor was only $50 I would rather change that before I go ahead and change the cats. Can anyone help here? Please let me know if you have any ideas on which O2 sensor I should change to try to get that code to go away. I don't care about having a code but I can't be getting 10 miles to the gallon or less. Please let me know thank you very much.
I have a 2000 Honda Accord ex with 6 cylinder 3.0 liter.

 

a cat isnt only bad when its clogged

 

P0420 = 'catalyst below threshold efficiency'

That just means whatever the engine is outputting is beyond what the catalyst can control.
Catalysts are not wear items, they can be damaged but normally they will not go bad.

If you have a code stored or the CEL illuminated you need to find out what it is and post up. One other note is being a speed density system the PCM can be a bit lazy with initiating all codes that may be the potential cause. And some codes will not tell you what failed but what the failure is. Treat the problem, not the symptom.

Several things that will cause the dreaded P0420
Out of tune engine. Worn, or even loose, spark plugs with incorrect gap and/or lazy HO2S'. O2sensors are the nth spark plug(s). When you replace plugs, replace the O2(s).
'00 V6 I believe is COP ignition. Pull the coils and verify the boots are not damaged, no rust on the coil spring nor oil fouling down the spark plug tubes.
Valve adjustment has not been performed in a while causing a misfire. Unburnt air/fuel into the exhaust will cause P0420 issues, if too much fuel the catalyst can be damaged from fuel igniting and overheating and melting the substrate.
EGR system partially plugged. Check individual ports and the main feed port, fairly common for these to gunk up. This will cause a lean misfire at cruise speeds, sometimes it's very noticeable, other times it's imperceptible. Incomplete burn of air/fuel and into the catalyst. It can only do so much.
Make sure there are no false air leaks, failing gaskets, rotten vacuum hoses, verify PCV and PCV valve are operating correctly.

Cleaning the EGR ports and adjusting the valves on a J series V6 doesn't even cost you money in parts. Gaskets are all reusable. And the little window on the cam belt cover shows when a given piston is at TDC. 3.5 engines actually have a number on the cam cog for easy valve adjustment.

Bank one, IIRC, is actually the firewall side. 1-2-3 are on the firewall side(bank one). 4-5-6 are on the radiator side(bank two). 1-4-2-5-3-6

 

There are many questions to ask here...

How old is the gasoline in the car?...

When was the last oil changed?...

Do you smell anything at idle?...

Are there any overheating issues?...

There might be more questions, but that's what I want to know first.

 

The gas is new

Last oil change 1000 miles ago.

And no to all your other questions.

And I just changed cat and spark plugs and code came rite back.

 

Is the vehicle in fuel control/closed loop operation? If it isnt it can cause the cat to not work properly and set this code.

Youll need a scan tool with at least generic data available.



Or you can keep throwing parts at it til the code(hopefully) goes away.......

 

how do I know or tell if the vehicle is in a controlled / closed-loop operation

 

Scan datastream