Have a '03 Jetta with a ECU that doesn't want to read the ESS consistently. Replaced the ESS and checked the wiring, still drops the ESS once in a while especially when hot. Next suggestion was the ECU, so I picked up a used ECU without bothering to read up on SKCs, IMMO-3 and what one has to do to get a used ECU to work in an 03 Jetta. So now looking for the best (i.e. least expensive) option for reprogramming a used ECU to work in my Jetta.