I have a compounding problem on my 1999 Accord LX automatic:

About a month ago, my car broke down due to a failed a heater hose and the engine overheated to the point that the spark plugs melted. Replaced the hose, passed the pressure test - so no engine damage. A few days ago, the check engine light came on: P0420 "Catalyst System Efficiency Below Threshold." My assumption was that the immense heat that had gone through the exhaust system had fried the O2 sensor and/or damaged the cat. I was going to repair this weekend. HOWEVER today I noticed a loud chatter or vibration noise coming from beneath the car - sounded like something loose. This occurred when I started up and the car was in idle; once I hit speed I didn't really hear the noise. Perhaps something in the exhaust system as well?

Any suggestions what to go check? Do I have one problem or multiple problems? Any technical input is greatly appreciated.

 

bang on the catalytic converter. if it sounds like a can of marbles you will need to replace it

see this thread for help:

https://honda-tech.com/forums/honda-accord-1990-2002-2/99-accord-code-p0420-cat-conv-need-help-3068452/

 

I recently got that code on 99 civic lx i checked the headers and there was small crack in it. I took it to a mechanic and he said there were made out of a cheap material and it developed a crack and due to heat a pressure it did a blowtorch effect on the inside of the cat. I suggest either replacing the cat or go about hollowing it out so your cel goes away. What I did