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I swapped out my old thermostat with a Lyric T5 around Christmas time. I live in an older house so wasn't sure I could make it work. I opened my old thermostat and saw that there was a blue wire that wasn't being used. I went down to the furnace and saw it was hooked to C. I swapped out the thermostat and used the blue as the common. Thermostat wouldn't turn on. I couldn't figure out why it wasn't working with using the blue wire as the common. I looked online and saw that I could use the green wire as the common and then use a jumper from the fan to the ac. https://www.youtube.com/watch?v=BftoUCnKYls is the video I watched. This got the unit working and I thought all was well. I checked the AC and I was getting air from my vents and thought I was fine. I didn't think to check the unit outside. Fast forward to now and it is getting warm and I turned the AC on one day to cool it down and then realized the AC outside wasn't actually turning on. I have no idea what could be causing this. Any help would be appreciated. I can upload some photos if needed. Thanks for any ideas. Link to pictures here

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    You're going to have to show us the wiring. We need to know about the wiring at the thermostat, as well as the wiring at the furnace/air handler. If you can't get clear photos, include a diagram that shows how everything is wired. Including the make and model of all the equipment, can also be helpful. – Tester101 Apr 10 '17 at 11:43
  • I consulted the Lyric website and they suggested using the green wire as the common and then a jumper if you don't have a common wire free. Since for some reason the blue wasn't work as the common I treated the situation as me not having a common wire. – Mike Apr 11 '17 at 0:56
  • Pictures have been added to the original post – Mike Apr 12 '17 at 2:34
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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.

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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.

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