Hello,
We installed the Honeywell RTH230B thermostat this January and it worked fine with our gas furnace. The problem is when we switched the control to cool yesterday the compressor turns on but the fan won't work in auto mode. It does work if I turn it to on mode only. Any ideas why the auto mode won't work? Thanks!

There is an internal jumber built into the thermost that sends power to the blower relay at the same time it sends power to the AC contactor. So if your furnace or air handler is new enough then you probably have a printed circuit board, and if you do some boards have hard wired relays and some have plug in relays. You need to find the relays on the circuit board and getnly tap each one to see if any thing happens. It is possible that there is a cold solder joint and by taping it you might cause it to make contact and the blower might come on. If you have a door switch you will have to make sure the switch is depresed to make this test, you can depress it by putting some duct tape over it temporally. Also there should be test points where you can short across them and each operation should come on indivigualy except for the gas on mode.