Can't get the AC compressor to turn on after switching to Lyric T5 Thermostat

I had the same issue with the ac not turning on. I had 4 wires on the old thermostat. I tried both methodes of using the yellow or the green as the common. Nothing worked.

Solution- the wiring had the 5th wire still available but not used. I connected the unused blue wire to the common in the thermostat and the othet end to the common on the furnace. AC finally kicked on. You need all 5 wires for a heat and ac system.

Hi i just want to say my husband struggled with this for about 16 hours. Our thermostat Did have 6 wires he connected four of them at first, then he tried each cable to see if they would work as the c wire but they didnt. The next day he went to the store and bought a 24v transformer. He got it to turn on but no air no fan no nothing would tuen on. What he tried next is what the thermostat said it didnt need. He made both red wires touch(jumper) got it working but we got only heat he messed with settings and got cold air only. He wasnt happy with that setup, he went up to teh roof and it was quite simple to see how the wires were connceted we did have a brown cable connected as the c wire. He ended takinh off everything and tried the brown cable as the c wire but he was sure he had tried that. Then he thought about it, he used the red wire as the rf wire and left the r wire empty. That did it. Remember the second red wire came from the transfomer he installed where the doorbell was at. He took that one out so we were back to having only the wires coming from the hvac unit. So i guess RC wire was required but not the R wire.