These sensors aren't very reliable, but they can be recalibrated, and calibration may be your problem. You can find a manual, including calibration instructions, at:
I attended a seminar at the Salon WBCCI International and one of my notes to myself, for what that is worth, sez: "Sensors can dry out, especially problem in black tank (debris) but usually enough liquid left in tanks after dump it won't likely dry out." You can get cleaners to put in your tank, and/or you can try the ice-cube trick - put a lot of ice in your tank, drive around so it sloshes good, drain. But my guess is you need to recalibrate.
My black isn't too bad but my gray tank tends to go from Green to Red with little time at Yellow; in fact, Red seems to be simultaneous with gray water backing up in the shower
p.s. - I just noticed the above manual sez the unit is warranted for 5 years, so check that out before paying for any replace/repair on your own.