A scan tool is a very useful tool and must be used when referred to throughout this section. In addition to displaying DTC(s), the scan tool is capable of displaying input status and data values the heater and A/C control system uses to perform certain functions. It is also capable of sending specific commands to system components to determine if the malfunction is due to the heater and A/C control unit not sending a proper command, or the component that received the command is not responding to the command. Using the actuator test in Special Functions as an example, sending a message to the defroster valve actuator door to cycle from 0% to 100% and to monitor the status (ACTIVE/INACTIVE) can determine if the heater and A/C control can actually perform the function. If the defroster valve actuator is functioning properly and the heater and A/C control unit is still unable to operate the actuator, then the problem is most likely with the control unit.