As with any experiment, there are a number of things that can go awry. This page offers some helpful advice for common Seebeck troubles, but is in no way an exhaustive list of every possible setback. The number one rule when all else fails; try turning it off and on again!
If there is a problem with the equipment that you cannot solve, try contacting Dr. Robert Kykyneshi who has extensive experience with the apparatus, or PH/CH electronics person Mark Warner who has fixed this malfunctioning multimeter before.
The pdf version of the DM5120 manual can be found here.
This machine is starting to become very old so information is hard to find online. The User Manual is on the desk in the Seebeck Lab and is a great tool. As of now the largest known error to effect the Keithley 195A is the OFLO error. This error is telling you that the input is outside of the Keithleys range preserving the sign of the measured overflow. There are several methods to fixing this error that I will list below.
1. Double check the connection to the computer at the back of the Keithley.
2. Turn the device off. After 20 minutes or more turn the device back on and wait the 1 hour warm up time.
3. (This one might just be luck but it's worked twice for me now after following the above steps) Ensure that the Keithley connected to the Dim 5120 PDMM is turned on. It's not connected to the Keithley 195A at all but I've noticed it works. I would guess that Lab View expects that DMM to be there and there are issues if it's not turned on. This is just a hypothesis though and needs further testing.
If none of these steps work for you please feel free to reach out and contact me (Aaron Dethlefs, Dethelaa@oregonstate.edu, Dethlefsa@gmail.com)