The book says
Air trapped in the system
Fluid level too low
Fluid mixture too weak or strong
Circulation pump not working
Sensor defective
12v supply not stable
I had a similar message recently. My battery had gone flat overnight. . After turning off the 12volt supply and charging the battery the message disappeared (after resetting the heating control to reset). . Two weeks later my battery overheated (while we were away in the caravan on electric hookup) . So the battery was on it's last legs.