2000 Infiniti I30 Infiniti I30

Hello- I recently had my muffler changed and the pipe connecting to the catalytic converter replaced. The service engine soon light was on and the car was idling high so the mechanic reset it. It came right back on and he found code 505 on it after he put it on the machine.

The next day the car would not stay on unless I hit the break first. Today the car would big stay on unless input it in neutral, pressed the break and the gas - then put it in drive. I was able to get home but had to drive with both feet. (Car is an automatic). Once home my father turned down my idle And the car turns on and runs ok. What on earth could be the problem? Please advise!