2003 ford focus running rich

my brother in law has an 03 focus thats modifies pretty good and it's running real rich. I think it's the oxygen sensor because the high flow cat has been hollowed out. why i don't know but it was that way when he bought it. i just want to knw if anyone has any other ideas on what it could be. it has an aftermarket chip in it but i don't think thats the problem. but if someone has another idea please don't hesitate to say something.

Pull the chip and see if its still rich. If it's not then you need teh chip recalibrated to his mods.

well the hollow cat shouldent make any difference as the O2 sensor after the cat is just for checking the emissions system, the first O2 sensor is what actually sees the level of oxygen in the exhaust to adjust the amound of gas injected, and when they fail, they usually cause the car to run rch, it's an easy fix assumingit's the problem, just feel along the exhaust pipe find it unscrew it and screw the new one in, and if that was the problem, then your ready to go, but i doubt that the cat could cause any issues with the O2 sensor