A month ago my T8200 Video Doorbell stopped working. I deleted it and tried to re-add it however it fails on the Voltage Diagnosis indicating my Transformer is not capable to power my device. It has been working fine since I purchased it in 2020. There was a firmware update earlier this year. Nothing else has changed.
You can easily check using a multimeter
You’re transformer could have gone bad or has weakened it’s output power, especially the VA _ voltage amps.
Maybe the connections on the rear of the doorbell have loosened or on the output side of the transformer?
What are the specs on your transformer?
Used my multimeter and the voltage was 18VAC, however the when I used my amprobe the amperage on one of the wires was only 0.12 Amp (2.19VA). I replaced the transformer with a 24VAC at 20 VA. This solved the problem. Thanks jd164
Good 
, But… That transformer will eventually fail.(when, who knows), It’s suggested to use at least a 30 VA or even 40 VA.
The doorbell is drawing more current then the transformer can output. Thus putting stress on it.
I appreciate your insight however on a Sunday there were limited choices available. The 16VAC 10VA transformer I replaced worked for 3 years powering the Eufy doorbell camera so if I get another 3 years or more I’ll be satisfied. I also noticed that the original Nutone doorbell chime had not been jumpered to bypass so the transformer was also trying to power that device as well. This has also been rectified by installing the jumper across the Trans & Front contacts. Once again thanks for pointing me in the right direction to resolve the issue.
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If your T8200 Video Doorbell has stopped working and you’re experiencing a voltage diagnosis failure during re-addition, indicating an issue with the transformer’s capability to power the device, there are a few steps you can take to troubleshoot the problem. Check the transformer to ensure it is functioning properly and hasn’t degraded over time. Verify the wiring connections between the transformer and the doorbell, ensuring they are secure and undamaged. Regards