Most likely causes (in order of likelihood)

  1. Toilet paper or waste film coating the internal probes — accounts for the overwhelming majority of stuck sensors.
  2. Hard water mineral deposits on the probes (less common but real).
  3. Failed sensor wire or connection (rare — only after the above are ruled out).
  4. Damaged probe inside the tank (very rare).

Diagnostic steps (in order, free/cheap before expensive)

  1. Empty the black tank fully through a proper dump.
  2. With the dump valve closed, fill the tank approximately 2/3 full with fresh water through the toilet.
  3. Add a tank cleaning treatment — the most effective options are (a) 2 cups of Calgon water softener plus 1 cup of laundry detergent (the 'Geo Method'), (b) a commercial enzyme-based cleaner like Happy Camper, or (c) a tank wand treatment like RhinoFLEX flush.
  4. Drive the RV for 30 minutes or more — onto a freeway if possible. The sloshing scrubs the probes far better than any static treatment.
  5. Let the mixture sit overnight if possible (24 hours is even better).
  6. Drive again the next day, then dump and rinse thoroughly.
  7. If sensors still read full, repeat with a more aggressive treatment. The 'ice + water + dish soap' method (a bag of ice through the toilet, half-tank water, dish soap, then drive) is the heavy-duty version.
  8. If after 2-3 cleanings the sensor still reads full, the sensor itself may have failed — confirm with a service tech.
  9. Going forward, use RV-specific toilet paper or single-ply only, always use plenty of water with every flush, and treat the tank monthly.

DIY vs. call a tech

100% DIY. No sensor cleaning kit is needed for routine cleaning. The Geo Method is the longstanding owner-favorite solution and costs about $5.

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