Motor mount replacement is common, but I wouldn't think that would cause the P0420 Catalyst Efficiency code.

You don't mention mileage, but if you don't have a record of a valve adjustment in the past 60,000 miles, do that.

Owners in NYC would think that 19 MPG is OK, BTW.

 

Thank you! Im at 237K. I don't fight traffic much where I live so I'm used to get the 23 MPG as advertised. I'll check the valves

 

Wipe the throttle body clean, too.

 

Other things that can cause a P0420 code are loose or leaking gas gap, pre-cat exhaust leak, or evap problem I believe.

 

On my 2006, got the code when I had a leak due to rust.

 

Update for people who find this post later: I found this link in another page on this forum that provided a list of possible non-CAT causes for this code. Code P0420 Honda: Catalyst System Efficiency Below Threshold Bank 1 (With Video)

As I said in my original post, I was pretty confident the CAT was working because I use an infrared thermometer to check the temperatures. Exhaust temperature post CAT was about 150 degrees fahrenheit hotter, which is what its supposed to be. When the CAT was cool, I tapped on it a couple of times and it didn't sound like it was one solid piece, which would indicate that if melted, and it didn't sound like a bag of pebbles, which would indicate it had broken down into pieces.

I had replaced the spark plugs recently so I was confident it wasnt a spark plug issue, but I wasnt so sure about fuel or air. I used a 16oz bottle of Cataclean in my gas tank to clean the fuel system and any gunk in the emissions system, and I changed my air intake filter. While changing my air intake filter, I noticed that one of the hoses from the grille to the airbox had become disconnected, and that the hose from the airbox to engine had a leak in it. So i reattached the one and replaced the other, and then cleaned my engine bay with purple Super Clean degreaser because another commenter had said I needed to clean the throttle body.

So far car is running better. I cleared the code and it hasnt returned, but the last time I cleared it, it returned about two weeks later. I will update if the code returns.

It makes sense to me that the engine wasn't getting as much air, and was running rich, leading to slightly lower fuel economy. and worse performance.

 

this code pops for me every once in a while if I downshift and the revs stay high

 

@jbiedebach, mine did that as well but I used miracle oil and haven't driven nor family told me the CEL has come on. But I will keep using miracle oil for that reason to be sure it's not something as fuel mixture problems.