I have a '97 i30 and recently the check engine light came on. I got a code of 0504, which is the automatic transmission control unit. Is this something that is easy to replace or am I going to have to fork over some serious dough to fix this? The car otherwise runs well.

 

I got this code about a year ago when I had just installed my Stillen FSTB. I discovered a part of the trottle postion wire/sensor wire loom had rubbed through on the Stillen bar and was grounding out. This then caused the transmission control unit fuse/relay to blow, and made the transmission go into safe mode, locking it in 3rd gear. After I replaced the fuse, it blew again, so I had to cut the throttle sensor loom bunle apart, and relocate about an inch away using heat shrink and solder. Even for a car savy person this was complete PAIN IN THE A$$, and took many, many hours of troubleshooting and repair!!! The cost was minimal, maybe $30 for the fuses/relays, but it ate up a lot of my time.

Good Luck!!