A vending machine door monitoring system for an audit system is provided. This door monitoring system may utilize the same door switch utilized by the vending machine controller (VMC) in order to detect the state of an access door for the vending machine. This door monitoring system may have a separate power supply for applying a back-up voltage to a door circuit in the event that the VMC fails to supply the voltage. The door monitoring system may be further configured to detect a state of the door circuit regardless of the order in which the terminals from the door switch are coupled to the audit system.