Hi, I have removed my old dial thermostat and it had two wires connected to it red & yellow, a third wire, green was not connected but sticking out of the wall. I have just purchased a Honeywell 3200 programmable model and connected red to R and yellow to W.
Nothing works... do I need to connect the green wire? Please help.

Thanks

Two wires being used must be heat only unit. If it's a mobile home remove top door on unit. Should have a black switch on the left side. Turn it on if nothing happens check breakers.

R and W are the correct terminals for a red and white wire. It is possible the yellow wire is actually an aged white one. See the stickies at the beginning of H&C. Check the directions for the thermostat. It may need batteries or a connection to the opposite terminal of the transformer as the red wire for power. If so, you could reconnect the green for that function. The green is meant to be connected to the G terminal to provide a fan on option. The furnace may or may not be wired up to provide that.

I didn't check the stat stats, but some programmables need a ground wire (c). An old thermostat uses r and w the 24v comes through the r and returns to the w to signal the furnace to come on, this does not provide power for the t stat read out. Hence you need to ensure that there is a third wire to the c on the furnace mated with the c on the thermostat. Check your wiring diagram. Or get a battery operated programmable they require no exterior power and just the r and w will do. Side note: not all manufacturers use c as ground wire check diagram