Why has the receiver been saying running self-check in the blue screen for days?

Self-check for days is a bad sign. I would unplug from power for a minute or 2, verify coax is finger tight, and then plug back into power. Hopefully that memory clear will fix whatever got it stuck. Be aware that it will have to rebuild the guide. If that doesn't work, then getting it replaced will be needed.

Before calling for a warranty replacement or considering an "upgrade", I would get guidance first. What model is the problem box, and what are the models of any others you have?