Hi. I have a 2006 Infiniti G35 base sedan. 157K miles, car runs great.  Got a P2A00 code. A repair shop that I trust replaced the upstream O2 sensor twice, second time with a Denso, and still getting the code.  shortly after the 2nd sensor was replaced, my Y-pipe from the bank 1 catalytic converter came apart at the cat's flange. I had the y-pipe patched until I get it replaced.  Could an exhaust leak after the cat have been related to the P2A00? My mechanic said the voltages were good on the Denso sensor. If not related to the Y-Pipe, what would be the next steps to troubleshoot the problem? I'd like to get this fixed on my next visit because I have to leave the car when they work on it and it's a hassle taking public transportation to/from work when I drop it off.  Also, I have a scanner and it shows the P2A00 code but the check engine light is not always on.  does that mean the problem is intermittent or should the light always be on if there's a stored code?