Yes, a transponder check is a high-fidelity legal requirement under FAA regulations (specifically 14 CFR 91.413) for any aircraft equipped with a transponder, even if flying strictly under Visual Flight Rules (VFR). The transponder must be tested and inspected every 24 calendar months by a certified repair station to ensure it meets the technical standards for altitude reporting and signal accuracy. Furthermore, if the aircraft is equipped with an altitude encoder, the correlation between the altimeter and the encoder must be checked simultaneously under 91.411. In 2026, this is especially critical due to the ADS-B Out mandate; if your transponder is sending incorrect high-fidelity data, it compromises the safety of the entire National Airspace System by providing inaccurate positions to Air Traffic Control and other aircraft equipped with traffic advisory systems. Operating an aircraft with an out-of-date high-fidelity transponder check can result in significant fines and grounded operations, as it is a fundamental requirement for flying in controlled airspace.