FAQ n.44272

CS ACNS.D.ADSB.090 requires that a means is provided to indicate the non-operational status or failure of the ADS-B Out system without undue delay.

This can be an indicator dedicated to an ADS-B failure, however a single transponder failure indicator may be acceptable if the crew are able to distinguish between ADS-B device or function failures and other transponder failures using suitable troubleshooting procedures.

AMC 20-24 does not contain an equivalent requirement.