05' 300C RS issue

Ok so heres my issue.    Some how upon RS it triggered a check engine code and now the light is on.     I was going to just disconnect the +battery terminal and ground it to re-set the ecu but I dont know if that is such a good idea on this car.   I dont know if anything after doing this would have to be re-set.

I tried talking to the dealership but of course they were of NO assistance.       The interesting part about the wiring was the "MUX" wire in the ignition harness.      The bypass I used was a 555U and I took the circuit board out of one of his keys.   

Overall I really like the car.   The prewire for the RS/Alarm kinda sucked but everything is else is pretty easy.    I also did a AVIC N2 and I had to bypass the factory Boston amp.    I used the rear outputs to drive the woofers in the doors.(gotta love pioneer for have a low pass output to drive a sub from the headunit)   Then the dash and rear speakers are 2 ohms so I had to run those in series on the front 2 channels from the headunit.   It actually sounds pretty nice minus not having actual front to back fade.     

If anyone has any ideas as to how to re-set the ecu that would be great.   Or if anyone else has had this problem,  what caused it?     I have been installing for quite a few years now and I totally felt comfortable working on the car and to the best of my knowledge its wired correctly.    The only thing that I can even think that might be wrong is the "MUX" wire in the ign harness.    

Thanks in advance