The problem is most likely with the thermostat. You can check the remotes if you have a video camera. Just turn on the camera, point the remote at it, and record it while you engage buttons on the remote. A video camera will detect the ir flashing of the remote. Once you know the remotes are good (which they probably are, anyway), you will know the problem lies with the thermostat. It's possible that a connecting wire to the fans (likely the ground wire), has oxidation and is not making good contact, or is slightly loose (the power wires will vibrate, over time, at 20 hz and can slowly loosen tighten-down screws, although this usually takes years). I would inspect all connections to the the thermostat, and make sure they're clean and tight, and see if that rectifies the problem. Aside from that, the thermostat is possibly defective, or you have a bad/oxidized ground connection somewhere other than the thermostat. A bad or dirty ground would inhibit voltage and current flow, not allowing the fans to turn on. If the thermostat is still under warranty, have it checked out. Otherwise, the unit itself might have to be serviced. Good luck, and hope this helps.