Rep Power: 0

CEL came on in 4th gear at about 3500 rpms .. car bucked then light came on and hasnt gone off since... any ideas as to what could be the cause? just had cat converter and o2 sensor replaced...

 
Rep Power: 216

Go to Advance or Auto Zone and see if they can pull the codes for you. What CAT did you get? Those aftermarket ones are garbage and there's a reason they're so cheap. But yeah. Get the code(s) and post 'em up so we can help you out.

 
Rep Power: 0

i got the cat at an automotive center, 830 dollars to have the thing fixed... car stalled out on a main street, i changed the oil, started up again to make it to my house and my car wouldn't start at all when i went out the next day. had it towed to the automotive center, they did a complete diagnostic check, put in R&R cat converter and R&R o2 sensor. the CEL was on when i bought the car, was told it had to do with the killswitch, which im almost positive it doesnt... i'm at a loss.

 
Rep Power: 189

bucking and stalling are indicative of misfiring, so I would start by looking at the cap and rotor, plug wires and plugs as the first source. I recently ran into this on my own car and ended up finding that the previous owner had overtightened the cap screws and had broken 2 of the 3 of them causing the cap to be loose and misfire and basically do exactly what you described. Sounds to me like you mechanic does not know his head from his *** so lets start with the basic and get your car fixed together.