This is an older, and not a common at all unit. Here is what your looking at
External EPFS Lost
This alarm is generated if the position from the External Electronic Position Fixing System is invalid (i.e. no external GPS). Due to the fallback arrangement for the positioning sensor this alarm can be inactive for up to 30 seconds (during which the internal GNSS is used) before the alarm becomes active. If this alarm is active, constant or intermittent, do a check and make sure:
1. The signal wiring between the EPFS and the R4 transponder sensor port (1,2 or 3) is correct.
2. The correct sentences are input (GLL or GGA and VTG).
3. The sentences are received correctly by using the Raw Data mode in the R4 display, change the baud rate if necessary
No Valid SOG Information/No Valid COG Information
These two alarms are active if the AIS system does not have a valid SOG or a valid COG respectively from any sensor. The SOG and COG is based on the speed log data if external GNSS provided and at the same time valid heading is available. If speed log data is not available it is based on the GNSS (external GPS) VTG message. If valid VTG is not input from EPFS the SOG and COG is taken from the transponder internal GPS.
This alarm is normally active at the same time as an External EPFS Lost alarm and No Sensor Position In Use alarm. If not, closely analyze the messages provided by the EPFS, Speed Log and Gyro and verify it is complete according to IEC 61162-1 and does not contain invalid values.
What it typically means 1 of 3 things.
1. A bad GPS antenna.
2. A wiring problem (corrosion) between the antenna and the screen.
3. A hardware/software malfunction with the display
The wiring is easy enought to look at, clean up any corrosion you may find. There are no tests for the antenna, there are also no tests for the screen. You can try putting a new antenna on it (if you can find one) if the wiring checks out. Other than that there is not much you can do aside from replacing it, as these are obsolete units.