I had two of them apart and didn't notice anything obvious like charring or PCB/component discolouration. They were both quite dust-free, as their environment was clean. The fans were not seized; no rattling or loose components. I reseated all of the connectors I could find in case something vibrated loose or had some oxidization.
I've had similar problem with my Innosilicon T2T 30TH, it was working fine all on modes and after halving I reduced it from top mode (performance) to lowest efficiency mode. And after a month or so of mining it just turned off and nothing helped it come back, I opened to check what is going on, control board was fine, hashboards were covered in heatsinks so I couldn't really see but when I opened the PSU... That's where the problem was, something burnt there and damaged it (for unknown reasons), I am yet to repair that PSU.
My point is... Check everything, even the things you doubt will go down. Like imagine forcing PSU at 100% for months and then lowering it to like 50-60% and it dies -_-