Chosen Solution
I’ve just gotten my hands on a replacement fan to swap the original out which had a broken ribbon cable for this component. On testing the Switch without fan it boots as usual (which it had failed to do with the snapped fan cable seated), but on trying to power on with the new fan installed it behaves just as when the original broken component was in place… I’ve tested it again in removing the new fan, and it does indeed boot again so long as the fan is not connected. What’s likely to be going wrong beyond what initially was taken to be a fairly straightforward fan replacement?
My guess would be that having the fan connected introduces a short somewhere, maybe in the connector?
@sybyl double check the new fan. Make sure that it is identical and see if the connections terminate identical. I would definitely try a different fan first. You did not tell us if you had trouble with that when you had the old fan installed, or how the cable on the old fan snap. There is always a chance it may have damaged the board connector. Check those contacts and make sure it all matches without touching etc.
