Today I purchased a Honeywell RTH2300 Thermostat. Pretty simple I thought! Hooked up the Red Wire to the R terminal, White to the W terminal, Green Wire to the G terminal and the last color I had was a light blue wire, did not have a light blue terminal, but there was a O/B which I believe the O stands for orange and the B stands for blue and eve light blue? Nothing in the book says anything about a light blue wire but I had no choice. Anyway after the wires were hook-up I put the batteries in and connected the unit to the wall and nothing! I have no digial or led wording on the screen, there is nothing just a blank screen? Unhooked the new thermostat and rehooked up the old one until I can figure out what is going on.

Thank you for any ideas on what is happening?

Disconnect the blue from the b post., what was the blue wire connected to on the original/old tstat?
Most of the time the blue wire is the common wire, go to the furnace and see what each color is connectet to on that end. Also if you didn`t turn off the switch or off at the breaker box you might/probly have fried your transformer,, check and let us know

I'm having the same problem someone tried to install my thermostat but doesn't remember where the blue wire goes. When the unit inside shut off the outside unit is still going to where the line freezes. Please help!