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RV holding tank level sensors not working?

Do your RVs’ black water and grey water holding tank level sensors not work? If they don’t, then I’ll bet your tanks are smelling bad too! When you don’t maintain your RVs holding tanks properly, they start to smell bad, and the level sensors will stop working!

What usually stops RV holding tank level sensors from working is that poo, paper, oils, grease, and food particles stick to the holding tanks’ walls and floor. This will cause your RV holding tanks to start stinking badly, sometimes to the point that you don’t want to be inside your rig. It also stops your tank level indicators from working correctly. This can be a real problem and will greatly decrease the enjoyment you get from RVing. It’s no fun constantly worrying about if your holding tanks are getting full or having to deal with a stinky RV because the black tank is smelling!

Inside the holding tank, there is going to be poo, toilet paper, food particles, oil, and grease floating on top of the water in the tank. This stuff is going to start sticking to the walls and floors and to your tank sensors, stopping them from working. Let’s take a minute to understand how common tank-level sensors work.

How do holding tank sensors work?

Manual and Electrical Tank Sensors

Black and Grey holding tank thru-wall tank level sensor probes
Black and Grey holding tank manual tank level monitor

In the case of your tank indicators, what happens is that the crud starts to accumulate on the sensors in the tank. This is caused by not flushing the tank properly. More on that in a bit, but let me digress for just a minute and give you a little education about tank monitoring devices. Your tank indicators will usually be one of two types, probe type or capacitive type. The probe type is straightforward to understand. A metal probe of some type has been installed through the wall of your holding tank, and it physically senses when water touches it.

Or there is a device like the one on the right that is installed at the top of the tank that, again, senses when the water touches the probe. This one works because the level of water in the tank floats the little black piece at the end of the stem, creating an electrical contact. It then shows the level of water in the tank. But if you look at them, it is pretty easy to imagine poo, paper, grease, and oil accumulating on them, preventing them from working.

Capacitive Tank Level Indicators

RV holding tank sensors. Capacitive and thru-wall tank monitors shown
Rv holding tank Capacitive Tank level Monitor system

The problem is still crud accumulating in the tank with the capacitive type, but this time on the wall under the sensor. The capacitive type senses the level of the contents of the tank through the wall of the tank. It is a sensor that is glued to the side of the tank. It detects the change in capacitance in the wall because water is touching the wall of the tank. I should really say liquid rather than water because, remember, there is a ton of crud floating on top of the water in the tank.


The electrical properties of the tank wall change when there is a fluid (water) on the other side. This sensor detects this change, but if enough crud accumulates on the tank walls, they stop working! Think of layers of paint. As you use the tank, it fills and drains and leaves a little residue on the wall of the tank.

The way you can tell which type of sensor you have is pretty simple. If your gauges read in percent full, like 53%,67%,78%,84%, etc., you most likely have the capacitive type sensor if your gauge reads in larger increments like 25%, 50%, 1/3rd. 2/3rds, etc., you have the probe type or mechanical type sensor.

So I hope you can see how easy it is to get enough crud accumulated in your holding tanks to make your tank sensors not work. I will help you fix it, but first, a quick story…

Why do RV tank-level sensors stop working?

It was only used three times

Adobe Bricks drying in the sun

Everybody has a story, and the one I see most often goes like this, I bought a used RV from a fellow that only used it three times! I’ve used it a couple of times, but I cannot get the black tank to stop stinking, and my level sensors don’t read correctly! I’ve tried all the commercial treatments to no avail. The problem is very likely that the past owner(s) didn’t maintain the tanks correctly. In fact, the reason they sold it but didn’t tell you was that they couldn’t stand the holding tank smell! The problem is an accumulation of old poo and paper stuck to the walls and the bottom of the tank. I like to call this adobe, like adobe bricks. Even if you have owned your RV for a couple of years, you probably have waste accumulation in the tanks, and your sensors are sketchy.

commercial tank treatments don't work

Also a problem with your grey tank

This can also be a problem with your grey tank, where food particles, grease, oil, and fats accumulate on the tank’s walls and floor preventing the tank sensors from working. You won’t smell the grey tank very often because there are ‘p-traps’ in the system that prevent the odor from coming out of the sink drain, unlike the black tank on many RVs. When you flush the toilet, you get hit in the face with an acrid sewer smell!

Commercial tank treatments don’t work to solve this problem. I have a video below about how to solve the stink in your RV. I invite you to watch that very educational video. However, I’ll share the answer: simply borax and some sort of detergent like Dawn dish soap. I invite you to watch that video; you’ll learn all about what causes the smell and how to fix it permanently! You can also check out this shortcut cheat sheet on maintaining your holding tanks.

So I will assume you’ve solved the smell problem, but there is another problem: you have a considerable accumulation of adobe in your tanks. This is a problem because it can reduce the capacity of your holding tanks. It is also the primary culprit for making your tank stink and your tank level indicators not work in the first place! The fix is relatively easy. It just takes time and patience.

For more great maintenance ideas, check out these articles on my website:

How to clean your holding tank sensors

Getting rid of the adobe

Borax used in RV holding tanks to help restore level sensors

You need to soak the s!^% out of it. Pun intended! It takes some time to soak it out, but here is the prescription: Add 1 cup of borax and 1 cup of Dawn dish soap, and fill the tank full of freshwater. Let it soak for at least a week, and two is better! We are softening the hard poop layer on the bottom and sides of the tank. This can take some time, so like I mentioned, the longer you can soak it, the better. If the tanks start stinking while you are soaking them, add an additional 1/4 to 1/2 cup of borax. The smell will disappear nearly instantly.

Dawn dishsoap

So when you are finished soaking, take it to the dump station, dump it, and flush the tank. Doing this in the middle of the week, you’re less likely to find the dump station busy. Or have to be self-conscious about taking too much time flushing your tank while others are waiting to use the station. Don’t cheat on the flushing. It is important to wash away the layers that have been softened enough to flush away before you start the cycle again. It may take two or three cycles of soaking and flushing to get the tank back in sound working order. The flush is super important, and you need to do it properly.

Flushing your holding tanks properly keeps your level sensors working properly

Clear Segment with grey tank remnants
Grey Tank Remnants
RV black water holding tank rinsing wand!

To keep your RV holding tank level sensors working properly, let’s talk about how to flush your holding tanks. The grey tank is easy. Using a bucket or garbage can, dump some water down the drain after the tank is empty and the drain valve is still open to flush out the tank’s bottom. You want to do this because there is thick milky sludge that will remain in the tank and cause odor problems. Do this a couple of times, and you should be good to go. See the video I shared with you earlier in this article for detailed instructions on cleaning your RVs grey tank.

Built-in RV black water holding tank rinser connection

On the black tank, you must have some kind of flushing system. Many RVs have them installed in the black tank. If your RV does not have a built-in system, you need to get one of these hand wands. The one to the right is the best one for the money I have found today. It is essential to flush the tank properly to get it working correctly.

When you’re flushing the tank, leave the drain valve closed and fill the tank to about 1/3 capacity. Then open the drain and repeat. Repeat the flush cycle of filling it 1/3rd full and then dumping it till it is relatively clear, like this.

Example of the flush quality you should see when dumping your RVs black and grey water holding tanks

You must have a clear segment in your drain hose

Clear segment for drain hose

So you must have a clear segment in your hose to monitor the quality of the flush water coming out. It would be best if you filled the tank 1/3rd full with the drain valve closed, then dump and flush it again until the water draining is fairly clear.

Don't use your drinking water hose!

Any one of these works, and in the case of this one, you can flush water into the holding tank by attaching a hose. This is not as good for flushing the tank correctly, but it is much better than not flushing the tank at all! Do not use your drinking water hose to flush your tanks.

Depending on your holding tanks’ condition, you may have to lather, rinse and repeat for another week or two. This will continue to soften the adobe crud in the bottom of the tank and allow it to flush away.

Cleaning really bad holding tanks

The Old Salt and Ice Trick to keep your holding tank level sensors working correctly

Another prescription I’ll give you to help keep your RVs holding tank level sensors working is that after a couple of soak cycles, and the tank is still not draining clear, then you need to scrub the tank… No, you don’t have to put your arm down the john and scrub.

20 pounds of ice
coarse rock salt

Start with about 1-3 gallons of water in the tank. Use less water in smaller tanks, like a gallon or 3 gallons in a 30+ gallon tank. Now add twenty pounds of ice and 4 cups of coarse salt in a small tank and 8 cups in a larger one; the coarser, the better! Like old water softener salt or coarse kosher salt. Now go for a 20-30 minute drive and allow the salt and ice to scrub the tank.

Go for a Sunday Drive

Go for a twenty to thirty-minute ride and allow the salt, ice, and water solution to slosh around in the tank. Find a nice twisty windy road if you can. Otherwise, turn plenty of corners and make a lot of stops and start driving. Imagine the solution sloshing around in the tank and how you can drive to promote the water sloshing around! Within reason, of course. You don’t want stuff flying out of the cupboards! Then go dump and flush the tank. Again, depending on how bad the tank is, it may take a couple more treatments of soaking and then salt scrubbing the tank, but with some patience and persistence, you will be able to get the tank clean.

Don't use pellet salt

This will have the effect of scrubbing the bottom and walls of the tank. It is best if you do this after you have soaked the tank at least once to soften the crud on the bottom of the tank. Don’t use pelletized water softener salt. It won’t work. We want the sharp edges of the coarse salt to help scrub the tank’s walls and floor.

How to prevent it, flush the tanks properly.

It’s as simple as flushing your tanks properly. That’s it. Properly flushing the tank rinses all the crud and mess out of your tanks and will keep your level sensors working. It has the lovely benefit of stopping your holding tanks from smelling, making your RVing experience so much more enjoyable.

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