Hi -
On the performance chip, does he have two different ECU's? They will be little boxes, computers if you will. Sometimes people buy one already chipped. Otherwise, the ECU has been reprogrammed and depending on the chip he has it depends what you can do.
I would find out what the codes are, they are stored even after the light goes out. You can go to a pepboys/autozone/checker and get the codes pulled.
If you search for the codes online or post them here then you should be able to get answers. My guess is the 02 sensors are bad, it's pretty common, as well as the diverter/bypass valves could be shot, Mass Air Flow sensor. All things that are known to be issues, especially as the car ages.
On tires, you can check with performance wheel and tire, or check craigslist. One word of advice, the car is AWD, all 4 wheels/tires need to be the same diameter and have equal air pressure or you will cause excessive wear on the AWD system.
I usually would recommend against replacing just two tires unless you can guarantee that the ones you find are at the exact same wear levels as the other two you will continue to use.
Give us a little more info and hopefully we can get you some better answers. Good luck!
