Quote Originally Posted by Chevy366 View Post
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.
Current and future ECM designs can/will be fast enough and smart enough to detect the presence of such boxes...at its simplest, the ECM could simply calculate the acceleration rate and if the truck is accelerating too quickly to be plausible (IE, the engine is making over stock horsepower), it will know something is up.

Harness/piggyback controllers are not the answer, unfortunately.

Ben