A connection can pass voltage fine but when asked to pass enough current for the use it can crap out.

A few weeks ago I was "brought in" on a real "tricky" one. Two other electricians (I use that term lightly in this example) could not solve this brain teaser issue on this commercial boat.

"Switch is good, voltage is at the plotter but plotters shut down." "These plotters are junk.".

They were blaming this on the $6000.00 worth of Garmin plotters.....

Look at boat, look at switch panel and location, apply alligator clip jumper across back of switch, plotters worked fine.. Bad switch. Reach in bin, pull out Contura switch, snap into place, wire it, done....

Just because something passes a voltage or continuity test does not mean there will be enough voltage/current at the source needed to drive it.. This was an easy one because it was a lobster boat and no switches last on them no matter how water proof.......

Owner: "That's it, its fixed?" "Those are supposed to be waterproof?"

Me: "Waterproof on a lobster boat ha, ha ,ha, ha, ha......... "

He now keep's a spare switch on board. I also gave him an alligator clip jumper, because this won't be his last bad switch seeing as he hoses the dash, plotters & engine panel with salt water...........