6 month old Nest (3rd generation) is determined to not work

[SirPickyPaws]

I installed the thermostat in March 2023 and was thrilled to have an option for a smart thermostat that didn’t require a C wire. It worked perfectly until last weekend when the screen went black and I couldn’t turn it on. Tried recharging it through the USB port. Nothing. Black screen, wouldn’t turn on. Bought a 24 v c wire transformer. Now the unit turns on but says there’s no c wire installed.  I’m at a loss. 

 

[Patrick_Caezza]

@SirPickyPaws 

Did you buy a 24vac transformer?

[SirPickyPaws]

@Patrick_Caezza  I did. I read somewhere that those only work on heat only systems? I have a Nest downstairs that works fine. Brought it upstairs to try and it works fine. I’m wondering if it’s a defective device?