Testing the line voltage and control voltage can be done quickly, and the problem is often found while performing these first two tests. Line Voltage To The HVAC EquipmentIf the system does not operate at all, check to see that it has the correct line voltage. Transformer And Control VoltageMost residential equipment uses a 24-volt control transformer. Secondary Voltage CheckCheck the secondary voltage immediately after power-up anytime a control voltage fuse or transformer has been replaced. Observing what components were operating when the control voltage dropped will help isolate what parts could be causing the problem.