I had the same problem with my new wifi thermostat. Thought that it may help to someone who has similar issue with my old unit.
So this was my problem. Here most of the people explained new furnace units with control boards. My unit is one of the oldest units which doesn't even has a control board. It only has power supply (transformer) and wire connections. While i was trying to figure out whole system, found attached furnace wiring diagram.
As you may see from diagram there suppose to be a c wire at my furnace transformer. Checked transformer and traced each cable which goes to thermostat. Even though there were spare wires coming from thermostat cables, nothing was connected as c wire. There was only green, yellow, red and white color cables were connected. Found c wire connection on transformer and connected one of the spare cable which goes to my thermostat.
Finally i was ready to hook up my c wire to my new thermostat to work it. But nothing happened. Checked transformer red and c wire connections with my multimeter and there was 24v ac. Really confused and started to think about having faulty unit option. Right before dumping my thermostat, remembered an important detail. What if i miss something at my thermostat cabling. Watched tons of videos about how to work it with an external power supply. All videos has a simple common point which is; there suppose to be a "-" and "+" polar to work any electronics :)
I was assuming that red and c wire are enough to work unit but it wasn't working. If you have an old unit like mine and if you don't have seperate "Rc" cable, u need to add jumper cable between "Rc" and red cable as it is stated at unit manual. After i add jumper to thermostat, finally could work it.
By the way; these units come with factory default settings. If your fan, ac, heater doesn't work don't panic. Just rearrange your thermostat setting from the menu according to your furnace system by fallowing mannual.
Be Carefull!!! If you do not have enough knowledge about electricity or electronics, please don't risk your life and don't touch your system. If you still want to do it, don't trust anything like furnace cover security switch (which shuts off the whole unit when you open front cover), just work when you turn off home furnace main switch.