If this rig has been sitting unused for a while, I’ll bet it is simply a connection problem between board/components. If you have the KAT3, try reseating the coax cables connected to it…both ends. Same with the KSYN3A and KRX3 (if you have it). Another batch of connections that might be the culprit can be reseated by removing the front face and carefully replugging it. I believe the K3S may have gold pins there but it is worth a shot. Lastly, you might reseat the KLPA3A board (the little square board inserted in the main board accessible from the bottom I think). I had this problem once on one of my three K3s and it was scary until I fixed it and it never happened again on any of them. My money is on the KLPA3 and/or the KSYN3 being the culprit.
Hopefully, someone else will give you a better answer since I’m basically an appliance operator(engineer wannabe) but these would be where I would look. Probably in reverse order as I listed them now that I’m thinking about this a bit. 😉
Waiting on a pair of K4HDs