A microprocessor controlled system for ensuring the safe operation of an auxiliary device, such as a wheelchair lift used in conjunction with a vehicle. The microprocessor preferably allows operation of the lift device when one or more parameters are met. These parameters include the following: the vehicle is in "park", the parking brake is engaged, the vehicle ignition is "on", the lift power switch is "on", and the lift door is open. The present invention does not allow the vehicle to be shifted out of park if the lift door is open and/or when the parking brake is "on".