It fools the ECU into thinking the engine is cold.
So adds more fuel even though the engine is up to running temp.
Why is this bad?
Unlike a remap, this just adds more fuel, but doesnt alter the ingitions timeing to match.
Worst case senario?
deternation
Dont know enough about diesal engines to comment though.
I know someone who is useing one on a 1.4TDi Lupo, and has had no problems.
But he hasnt got anywhere near the 30hp the advertise.
He thinks he may have an extra 10hp at the most.