New Nest thermostat installed, but AC turns on when heat is set

Security Camera Brands Forum Nest
1

I installed a Nest thermostat on my 4-wire system (Red/R, Green/G, White/W, and Blue/Y). I have a forced air furnace with central AC. I connected the wires exactly as they were on the old thermostat, but when I turn on the heat, the AC starts running instead. Any ideas on what could be wrong?

2

Can you post pictures of the wiring before and after? It sounds like something may not be wired correctly. Also, if you go through the Nest setup or diagnostics, does it show anything unusual?

3

EvansBrown13 said:
Can you post pictures of the wiring before and after? It sounds like something may not be wired correctly. Also, if you go through the Nest setup or diagnostics, does it show anything unusual?

I figured it out—I need a C wire, but I don’t have one.