PORTA.4 as analog input

[LinkMTech]

My suspicions to the problem is "A" and "C", definitely "C".
kblim, you need to buffer the O2 sensor output signal before sending it to the PIC. This keeps it from loading it down as you have seen when it almost worked on RA7 and RB2. I've provided an old circuit that I used back in the day of playing around with injector PW modification. It's basically the same thing you are doing but I used a pot where you want to use the PIC (before my eyes were opened ).
Good luck on the original RA4 "short to ground" problem.