heat

I'm at my last nerve with my 07 exl...I bought it 2 years ago and up until recently I had a vtm-4 light and cel p0420 on and off. Had the dealer I bought it from do a "cat test" said it wasnt the issue and couldnt diagnose because the light was always off by the time I drove there ?‍♀️ The most...

Any idea what fix this might entail?