The only time you have to test the encoder is when you have a reason to suspect an error in data correspondence. If you change the altimeter, transponder, or encoder or send any of them out for repair then you must comply with paragraph C of F.A.R. 43 appendix E. As long as there is no reason to suspect an error than you are only required to test your transponder every two years and not anything else.
FARs 91.413 and 43 Appendix F. apply. Cheers: Paul N1431A KPLU
