How can I test if my NAV4 is receiving input from my GPS unit?

Raw NMEA data sent from the GPS can be printed by entering NMEA test mode. This causes the NAV4 to output all received NMEA data. To place the NAV4 in NMEA test mode push the 'Y' button 3 times in quick succession (NAV4 starts to tick). The NAV4 will print a list of all the NMEA data that it can use. Cancel the test mode by again pushing the 'Y' button 3 times in quick succession.