The Alfa bug bit
Join Date: Jan 2008
Location: New Zealand
They are designed not to interfere with the normal ECU, so disconnecting it shouldn't throw any fault lights up. If it is, then it's possible that it's not wired correctly or is malfunctioning (or the car has a fault). When disconnected, does it run the same or can you notice a difference? The difference should be noticeable more on the motorway, in higher gears. It should have more low down torque....
Depending on the box you have, you may have a grey wire as well as a white wire, coming out of the box. The white wire is the signal from the throttle and should be variable. Between 1v and 5v, depending on throttle position but the grey wire normally connects to a on/off switch. Make sure the grey wire isn't touching earth, if it is, it's stopping the box working.