Water temp sending unit

[J-440]

  I have 2 water temp sending units mounted after the t-stat.  1 is for my FAST EFI system and the other goes to a dash gauge and they are both mounted next to one another.  The FAST unit is reading correct while the other one is about 50 degrees off.  I even took it out, tested it with a heat gun and got perfect readings.  Used some thread sealant and put it back in, started the car and got the same results.  This was after 15 minutes of idling.  It came in a kit with the right gauges also.  What gives?
  One more thing, do the t-stat and radiator cap need to match?  I have a 195 stat with a 16psi cap.  Thanks again.

[polywideblock]

 while I know your asking about sending units
   is the 50 degree different one your dash gauge       they are notoriously inaccurate compared to aftermarket gauges 

[J-440]

 Sorry I forgot to mention that all the parts are aftermarket gauges.

[crj1968]

can you swap one gauge with the other?

[J-440]

  Dang, note to self, always check the grounds before you ask a question on this forum.  Thanks for the quick responses.