Is this a Cali emissions car? That will determine which system it has. Those cars have an exhaust manifold that's flat across the top (relatively) with a sensor up high I believe.

The non-Cali emissions has something that looks more like an old school header pipe (under the decorative heat sheild) and that will have an O2 sensor by the radiator.

If you do replace the O2 sensor, make sure to get the Denso brand as that's OEM. Too many stories of cheap aftermarket stuff not being a reliable repair.

The P0420 error number indicates a low catalyst system efficiency. (Google results)

My daughter's '03 will throw this code after long highway drives. Then when she's back home and driving around normally the light will go out on it's own. While this isn't ideal it's not a park the car until fixed type of thing in my opinion.

 

Is this a Cali emissions car?

No it's NY emissions and definitely resembles the second photo.

Thanks guys. I'll try clearing the code first and running again. If the code comes back, I'll get a temperature gun for the cat. What degrees am I looking for if the Cat is ok? If the Cat is good, then I'll try replacing the O2 sensor first I guess.

This kind of a crazy question, but I saw on you tube somebody suggested that if you did want to clean the cat, to just soak in Dawn dish soap and water over night. So with these cars since the cat is connected to the manifold and if I was to remove all of this, at that point I wouldn't clean it. Rather I'd just get a new one. But I was thinking without removing, what if I sealed the tail pipe and I unscrewed the O2 sensor and just filled the whole exhaust system up with the Dawn and water until it starts flowing out of the 02 sensor hole, that way I'd know it's full? Leave it sit overnight, then drain, reinstall O2 sensor and run the car. Eventually it would dry out, no?
This is a theory, but what harm can it do besides maybe cleaning both cats? LOL 😆

 

Could be related as serious misfires can damage the cat. Good to soak the O2 sensors for a while before removal. Sure, liquid wrench, pb blaster. Most parts can be tested. Simple test for the cat is checking temperatures before the cat and after; should be much higher after. O2 sensors can be tested as well. See youtube.

 

This doesn’t answer any of your questions, but I would suggest clearing the codes, driving your vehicle for a few days, and see if any code(s) come back.

 

Did you fix code P0420 Bank 1 ?

 

Had this same issue on our daughters 09 TC LE w/2.4L engine w/252K+ miles. She bought it used back in 2023 w/238K miles. Code P0420 means "Catalyst System Efficiency Below Threshold (Bank 1)". This is pointing to the Upstream Oxygen Sensor located before the Catalyst Convertor.

This is the process I used to diagnose the problem.

1) Tested the Upstream Oxygen Sensor for resistance and voltage.
2) Removed the sensor and used an inspection camara to look inside the Catalyst Convertor for and obstructions.
3) Confirmed Upstream Oxygen Sensor was bad and replaced. (Problem solved)