What I forgot to mentioned - check the relay counter on internal DMM (if present).
I do not find the way how to check it through front panel, only remotely by DIAG:DMM:CYCLES?
For instance on my unit the DMM relay K102 is almost dead >10M cycles. Probably someone selected different ranges 10V/100V or similar switching problem
For details check "Chapter 5 Theory of Operation - Internal DMM" in service manual.
I needs to desolder relay and replace it by new one Omron G6SU-2-DC3
Because my unit has only one issue ( errors in self test 740/741/748 and etc) all were due to unlocked instrument, when I locked calibration all are gone.Battery has 3V.
Based on the relay counter I expect that unit failed calibration due to K102 relay.
There is another trap - if you checking basic performance of modules put into terminal some wire. Unit has been supplied with two modules. Both as faulty ones with marking which relay is faulty.
So I checked the odometer and maximum was 25 cycles.
DIAG:RELAY:CYCLES? (@101,102, ....)
So I checked them according to service manual and yes they seems to be not working.
So I expect that someone reset relay counter, but modules were almost new. When I measured the relay contacts all were OK, yes the problem was in terminals!!!!!
The guy which has been cheeking the modules make same mistake probably.
If you are measuring the screw, there is ope circuit, when you put there some wire it is OK