RV tank sensors reading wrong and black tank clogs: what to do

How RV tank sensors work

Most RVs use simple probe sensors: metal screws that pass through the side of each tank at different heights (for example 1/3, 2/3 and full). When liquid touches a probe, it completes a circuit and the monitor panel lights up that level. The design is cheap and reliable electrically, but it can't tell the difference between liquid and anything conductive stuck to the probe: toilet paper, grease, waste or a film of residue.

Some newer RVs use external sensor strips stuck to the outside of the tank, which read through the tank wall and aren't fooled by residue. If you have those, skip to the wiring checks below.

Symptom: gauge always reads full (or 2/3) when the tank is empty

This is the classic black tank complaint. Paper or waste is clinging to a probe. Grey tanks do it too, usually from grease and food particles from the kitchen sink.

Fix 1: Better dumping and a soak

1. Dump the tank completely at a dump station or sewer hook-up.
2. Close the valve and add water until the tank is at least two-thirds full.
3. Add a dose of enzyme tank treatment according to the label.
4. Drive. The sloshing on the road does a lot of the scrubbing for you. A day of driving between campgrounds is ideal.
5. Dump again and check the gauge.

Some owners add a bag of ice and water before a drive to scour the walls. It's a long-standing RVer trick with no harsh chemicals. Results vary, but it's harmless.

Fix 2: Flush the tank thoroughly

• If your RV has a built-in black tank flush connection, attach a dedicated hose (never your drinking water hose) and run it with the valve open until the water in a clear sewer elbow runs clear.
• If not, a tank flushing wand inserted through the toilet lets you spray the walls and probes directly.

Warning: Never leave a black tank flush running unattended with the valve closed. Tanks can overfill and push water back up through the toilet, or damage the tank. Never use your fresh water hose for tank flushing. Keep a separate, clearly marked hose.

Fix 3: Check the wiring

If the tank is clean and the gauge still reads wrong (or reads empty all the time):

• Look for corroded or loose wires at the sensor screws on the side of the tank.
• Check that the monitor panel has power and a good ground.
• With a Digital multimeter (auto-ranging, with DC amps), you can check for continuity between a probe and ground when the tank is filled past that level.

Symptom: black tank drains slowly or not at all

The "pyramid plug"

When the black valve is left open at a full-hookup site, liquid runs straight out and solids pile up directly under the toilet into a hardened mound. Eventually the toilet won't flush down cleanly and the tank won't drain.

To clear it:

1. Close the black valve.
2. Fill the tank as full as you safely can with water (watch the toilet; stop before it reaches the bowl).
3. Add enzyme treatment and let it soak for at least a day, longer for bad cases. Driving helps.
4. Use a flushing wand through the toilet to break up the mound.
5. Dump, then repeat if needed.

Avoid poking hard objects into the tank, as you can puncture it or damage the valve. Avoid harsh drain cleaners not labelled safe for RV holding tanks; they can damage seals and valves.

Valve problems

If the tank is full but nothing comes out when you pull the handle, the gate valve may be broken (the cable or handle has separated from the blade) or jammed with debris. If the handle moves freely with no resistance, suspect a broken valve or cable. Valves are replaceable, though it's a messy job best done with the tank empty.

Calgary tip: In winter, a "stuck" valve may simply be frozen. Valves and the drain outlet are often the first things to freeze. Don't force them. Warm the area gently, and consider tank heater pads if you camp in sub-zero temperatures.

Grey tank tips

• Wipe greasy pans with paper towel before washing them.
• Use a sink strainer to keep food particles out.
• Dump the black tank first, then the grey. Soapy grey water helps rinse the sewer hose.
• At the end of a season, a long soak with water and a mild dish soap helps lift grease film.

Dumping routine that prevents problems

1. Put on gloves. Connect the sewer hose securely at both ends.
2. Open the black valve first. Let it drain completely.
3. Flush the black tank if you can.
4. Close black, open grey to rinse the hose.
5. Close all valves, disconnect, rinse the hose and cap everything.
6. Add several litres of water and a treatment dose to the black tank so the bottom stays wet.

Winterizing your tanks

Dump and flush both tanks before storage, leave the valves closed (or follow your manufacturer's advice), and add some RV antifreeze to each tank and P-trap so seals and valves don't dry out or freeze. Full details are in How to winterize an RV for a Canadian winter (step by step).

When to call a pro

Get help if a tank is cracked or leaking (look for staining on the underbelly or drips), if a valve needs replacing and you're not comfortable with the mess, or if a sensor problem turns out to be damaged wiring inside the enclosed underbelly.