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I've done a lot of di2 repairs and yet to see a faulty component drain the battery. It is usually a damaged cable. Front derailleurs with the braze on mount supporting bolt not engaged onto the frame can be an issue.
A cable that has been pinched can easily short the rest of the system. Cables that don't insert into components and junction boxes with a nice clean click are also a problem.
My go to is to strip the bike completely and lay the system out on a table for inspection. If I don't find anything the I will methodically switch one part out at a time which takes a while until I find the culprit.
News of 105 going Di2 and disc only comes at a time when I am at the end of my tether with both.
First the piston seal in my shifter failed out of warranty and now the bike shop tells me there’s a component draining the battery but E-tube doesn’t specify which component.
Is there a way of sorting this without plugging a different FD and RD in sequentially? I don’t suppose it will be a shifter problem and seems unlikely a cable could drain the battery.