All GTN 650 and GTN 750 navigators marked as Mod Level 4 or previous mod level are affected.
An issue has been identified where the VOR/LOC/GS may stop functioning in the affected units. This will be indicated by a message on the GTN and the Nav frequencies will be replaced by a red X.
Service Bulletin 1559 provides instruction to inspect and return affected GTN 650/750 units to Garmin for a Mod Level change. Service Bulletin 1559 also provides optional instructions for marking Mod Level 5 on the nameplate (serial tag) of unaffected GTN 650/750 units.
Mandatory for units listed in Table 2 of this bulletin. This Service Bulletin must be accomplished by October 31st, 2016.
Optional for units not listed in Table 2.