What I've been told is all installed transponders capable of transmitting (unless fuse is pulled and properly flagged as "Inoperable" ) must be certified every two years by an FAA approved avionics repair or calibration station. Repair stations can calibrate the encoder, calibration stations are not suppose to, but often do. (I believe these are the correct terms.)
This, IMHO is one of the most neglected certifications in any airplane, experimental or certified. A good number of airplanes are out of certification on their transponders, or never have had them certified at all. I found out the hard way ferrying a RV-9a from Washington St. The transponder was reporting 31,000'! Needless to say I got a phone call when I got on the ground, and got an education on transponder certification and the requirements there of. All Minn. Center wanted me to do is turn it off! Needless to say I complied.
