I purchased a Honeywell T5 Smart thermostat for suspiciously little money on eBay. It was advertised as tested working.
When I plugged the thermostat in, it did not power up. So I opened it up on the bench. Putting 24 VAC across terminals R & C provided no current flow. When I put power across RC & C about 2 watts were being consumed continuously.
While powering RC & C I probed parts of the PCB to see where the power was going. After checking several areas I noticed the chip U5 that was labeled TI 5416A. I took this to mean it was TI TPS54160A a 1.5A 60V step down DC/DC converter. Using the datasheet, I determined the chip was getting an input voltage but was not providing an output voltage.
When I probed components near U5 (related to the datasheet typical application circuit), I found that D4 was shorted in both directions. I desoldered it and replaced it with a 1N5819. When I applied power to R & C, I still got nothing. Applying power to RC & C made the device power up.
Normally a thermostat should accept power on R & C; this thermostat was not, but if it would take it on RC & C. This was acceptable for me given my specific heating only application.
I wrapped the diode in polyamide tape and sealed the case. I provided 24VAC power across RC & C and put the heating circuit I wanted to control across R & W.
E-waste repaired to working order for 10 cents worth of parts. It feels good!