Have you had any problems with the SES light coming on after installing the catback,because of the lack of back pressure? My neighbor just installed a magnaflow catback on his car and now his ses light is on. His car is STOCK .

 

You should get the car scanned and figure out what the code is. The lack of backpressue will sometimes cause the EGR not to function properly and you'll get a P0400 code, that only seems to happen on the 96-97's occasionally. It did happen to me, but it was when I added headers and removed my cats. Putting the cats back on provided enough backpressure to keep the EGR happy. I'm not sure of any guarantee fix for this, but this probably isn't your friends problem if he just added a catback, I'd get the car scanned.

 

has he cleard the ecm of the codes to see if it comes back? i'd do that, then if it comes back i'd get it scanned. i supose it's possible they cut/burned the rear o2 sensor wires on installation, that would cause the ses light to go on.