Per the federal aviation regulations 14 CFR 91.413, all transponder-equipped aircraft must have their transponder tested and certified every two years, in addition to the biennial altimeter and encoder checks required by 14 CFR 91.411 and 91.217 for aircraft that will be operated under IFR. It’s important to note that these requirements apply to experimental aircraft as well.
Transponder tests per FAR 91.413 IAW FAR 43 Appendix F:
Mode AC Transponders - $125
Mode S Transponders - $150
Altimeter system and altitude reporting equipment tests per FAR 43 Appendix E, Paragraph A, B, & C.
Standard single altimeter & encoder tests - $150
Electronic altimeter/encoder tests - Time & Material
Static system test per FAR 43 Appendix E, Paragraph A
All prices are for standard equipment and mounting locations, some tests may take more time. Equipment/system adjustments and/or repairs will be extra.