RedneckwithaEuro307

I am new to Euro cars, I just bought a 2003 TT 1.8T non running. When pulling codes it came back with P0600 which is a super generalized serial data malfunction...I've seen there is a relay that powers the ECU but when looking it up its mainly advertised for the Quattro which i do not have. Any ideas are greatly appreciated...previous owner says the car would downshift at around 60 and shut off and when started it idles rough. TIA!