Quote Originally Posted by GMPX View Post
Unfortunately if you want every other module in the car to work then you do need the ECM telling them what is going on. Like modern radios in cars, you are stuck with what they offer because the screen used for the radio also has climate control, sat nav etc, pull that out for something else and half the cars functions are gone.
A friend had a piggyback controller on his diesel truck, he just unplugged the the PCM and plugged in the piggyback unit into the PCM, then the piggyback controller took over operations from the PCM. PCM never knew it wasn't controlling the truck, all other functions worked fine.

They do that a lot with motorcycles too, you unplug the injectors and use a piggyback controller to adjust the pulse width modulation to obtain rich/lean mixture, ECU never knows.