G500 GDU 620 rainbow screen - possible solutions?

My G500 has suddenly developed a rainbow screen https://marysun.com.au/assets/Garmin%20G500.JPG . The issue was intermittant last week, but now seems permanent.

There is a GDU voltage alert "GDU has low voltage" but not sure if this is because all other sysytems were switched off when I tested.

Many thanks for any constructive solutions or advice.