Your response is what I've been needing because it addresses the problem as a system. Unfortunately, a precise sequence of events may not be possible as I have poked around so much without keeping adequate notes. But, it
all started one day when my wife reported a strange noise from the kitchen. I found the noise to be coming from the
wine cooler door display which was flashing rapidly. I was at least smart enough not to order a new door. It proved to work normally when I connected it to a known power source.
Then I took off the back panel and found twin power supplies connected to a common break out board. The voltage for the door emanates from the BOB, so I decided the problem had to be related to it. There's only on IC on the BOB, and my thinking was that it was far more likely to be the culprit than one of the caps, resistors, diodes, etc.
I found the IC on ebay and waited several weeks. I installed it. No change. Then I started deep research into common failures of switch mode power supplies. That led to my replacing caps, then diodes, then transistors.
I had violated all the rules of trouble shooting by replacing rather than testing. That's not the way I've spent my life as a person who generally succeeds in fixing most anything around the house. But I was out of my element. I simply didn't have any way of testing, didn't know how.
So, I began asking one by one about this component and then that one. The idea was to learn how to test each component and then order parts based on those tests. I mostly got no help so resorted to replacement as a process of elimination.
(At this point the only things I haven't replaced (on the two PSUs and the BOB) are four zener diodes, two for each PS. They are ordered.)
After each attempt at repairing the boards, I'd hook it all up to find that it still wasn't right. The last attempt had the fans for one Peltier spinning slower that what I would expect while the other set of fans would pulse off and on. I could hear the faint sound of switching from both power supplies. I only just now learned that the switching sound comes from the power transistors.
I did test both Peltiers with a 12v battery charger and both would heat/cool in a way that would seem normal based on what I understand about them.
And that, sir, is my story. I hope I haven't left out anything important. With regard to finding success stories that I could benefit from-- not so much. I've found a few old posts about dealing with failed units, but the posters usually lose interest in the subject once they've found their problem (or have given up on it).
As previously stated-- it ain't about the money. I've already spent more than the cooler cost. I'm just obsessed with seeing it through. After that I'm going to find myself a good electronics project with documentation so that I can go from start to finish without having to work blind.