My wife replaced our thermostat with a digital one by the book. It worked periodically. She must have crossed wires though because now it doesn't work. The box said "easy 15 min install." She got ambitious and tried it. Now I have the old one back on and still, no power to the central heating or a/c. Can you give me any guidance for next steps? The Honeywell help center is currently closed. Thanks so much.

Shut the power off to the a/c unit,connect the red wire (low voltage,the one that goes to the tstat) and the green wire and see if the blower comes on. Let me know what happens.

Thank you very much. I did as you said and there was no response. I have attached some photos of the wires and the thermostats to hopefully make this easier. What is my next step? Again, I really appreciate the help.

Sorry I couldn't get back to you early, it looks to me that you probably blew a fuse. Follow the red wire that's coming out of the transformer and see if there is a 5 amp or 3 amp fuse usually they are yellow or blue or check the transformer to see if it has a switch its usually on the side. If you have a voltage meter check to see if you have 24 volts coming out of the transformer, one side its high voltage. Let me know what you find. Next time you change a tstat shut the power to the a/c unit

Check theses specs to insure this is the correct tstat for you Shop Honeywell 5-2 Day Programmable Thermostat with Backlight at Lowes.com Have you tryied to reconnect th wires yourself to confirm wife's wiring. A common mistake is to fail to jumper the two reds on the stat

Thank you for the input. How do I know where to find the fuse(s)? I have searched the furnace and cannot find any. Could they be outside the house? I can't tell you how much I appreciate the help from both of you. I'll wait for your response. I understand if it takes a little while for you to respond.

It is the transformer. How hard is it to change myself?

Do you have 4 wires connected to the transformer? Its it 2 wires on each side?

Make sure the power its off!

Most my research points to a bad furnace transformer. Are those hard to DIY?

There are no fuses.

Do you have a voltage meter?

No and that's another reason to me I should just have someone out to fix this. Thanks for your help, I really appreciate it.

You should get 1 for yourself. A decent digital voltage meter should cost about $60 or $80. A transformer,about $25 to $30. If you decide to get yourself a meter let me know. Ill be more than happy to help you!

Next time shut the power to the unit before you do any work. You probably grounded the transformer.

Thanks Rodri.

Anytime!