Oceanic Atom 2.0 Error code

[Dave1952]

After having the battery out of my Atom 2.0 for about a year because of COVID, I put in a new battery, but when I hold the "M" button down to get the NORM screen I get a flashing code: "CKSM". I took the battery out for 24 hours and put it back in and still got the same result. Any ideas on how to fix or reset it?

 

[runsongas]

checksum error, contact oceanic

 

[Centrals]

Off topic
I routinely switch on the computer if it has not been used for a while just in case. Do not need last minute surprise ie just before a trip.