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.
The NAV4 buffer fault report is a standard report produced by the NAV4 if the print buffer overflows. This is not a fatal error report and you have no need to worry. The buffer fault report indicates that your NAV4 has noticed a printing error when recalling a NAVTEX message from memory.
This type of report is however rather unusual, if a GPS or instrument system is connected it could also be related to the amount of NMEA data sent to the NAV4.
The reason could be a weak signal. Weather forecasts tend to be longer than navigation warnings and as such can be more susceptible to 'drop up' of radio reception than the shorter navigation warnings. A count of 16 consecutive error characters will result in the rejection of a message by the NAV4.
There are 3 allocated NAVTEX frequencies:
518 kHz is the main frequency, this is used World-wide printing English language information.
490kHz is allocated for local language use.
4209.5 MHz is allocated for 'Tropical or long range use'.