Mark

ATC transponder unit with self test lights not working when button pushed.
  
     
  
I suspected a air/ground sensing issue which seems to be the norm for numerous systems on the aircraft (boxes that shouldn't be tested while in the air).

On the 767 (at least this -300 aircraft) two air/ground relays provide the control panel and transponder an electrical ground signal when the aircraft is in a "weight on wheels"  (on terra firma) condition.

Relay "K143" for the left system. "K200" for the right. My problem was right, but the print below shows the left.
  
     
  
Bad relay "K200" which kept the right panel/transponder in the air mode.
  
     
  
After replacing relay.....