FAA 91.411 & 91.413 testing & certification

All transponder-equipped aircraft, both VFR and IFR, must have their transponder tested and certified every two years (see FAR 91.413). Aircraft operating under IFR must also have their altimeter(s), encoder and static system tested and certified every two years (see FAR 91.411).

In addition, FAR 91.217 requires that all transponder-equipped aircraft have their altimeter and encoder checked for data correspondence after initial installation or following any subsequent modification or maintenance.