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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 smell 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 greatly decrease the enjoyment of 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 smells!

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 will start sticking to the walls and floors and 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, the crud starts accumulating on the tank sensors. This is caused by not flushing the tank properly. More on that in a bit, but let me digress for a minute and give you a little education about tank monitoring devices. Your tank indicators will usually be one of two types: probe type and 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 on the right installed at the top of the tank that, again, senses when the water touches the probe. This one works because the water level 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, it is on the wall under the sensor. The capacitive type senses the level of the tank’s contents through the tank’s wall. 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.

Multimeter

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

How 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 accumulate enough crud 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 several 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 few years, you probably have waste accumulation in the tanks, and your sensors are sketchy.

Get everything you need to maintain your holding tanks from my Amazon store and support the blog. https://amzn.to/3TERzea

commercial tank treatments don't work

Also, there is 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, unlike the black tank on many RVs, there are ‘p-traps’ in the system that prevent the odor from coming out of the sink drain. 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 wait 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 empty tank 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 RV grey tank.

Built-in RV black water holding tank rinser connection

You must have some kind of flushing system on the black tank. Many RVs have them installed. 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

Get everything you need to maintain your holding tanks from my Amazon store and support the blog. https://amzn.to/3TERzea

You must have a clear segment in your drain hose

Clear segment for drain hose

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 dumped and flushed it again until the water draining is fairly clear.

Don't use your drinking water hose!

Any one of these works, and in this case, 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 the condition of your holding tank, 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, you need to scrub the tank… 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-plus 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! For example, 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, 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 can get the tank clean.

Don't use pellet salt

This will scrub the tank’s bottom and walls. It is best to do this after you have soaked the tank at least once to soften the crud on the bottom. 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.

Get everything you need to maintain your holding tanks from my Amazon store and support the blog. https://amzn.to/3TERzea

How to prevent it, flush the tanks properly.

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

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24 thoughts on “RV holding tank level sensors not working?”

  1. Watched the video and read your text. Two questions:
    1) my sensors are “shot” clogged , whatever they don’t read out. So you say fill the tank , maybe 2/3rd? but I can’t tell when the tank is full / 1/2 full or whatever. How do I know while still in the process of soaking / cleaning? I can tell when the gray water tank filled – it backed up into the shower! Don’t want to use that method for black water!
    2) we are pretty much full time in one spot (trailer) and can’t easily run around the block to shake / stir. There is a tank flush fitting. would this be adequate?
    Thanks

    1. You’ll need to soak them. Do you live in the trailer, or is it a vacation home? If you leave it in one place and are gone for a couple of weeks, then add the borax and dawn and let it soak till you return to use the trailer again. Of course, don’t leave water in the tank if there is any chance of it freezing. The method works very well, sitting static. It just takes more time!
      Yes, you should find out if the flusher works and use it! As far as telling when the tank is full, if you open the toilet valve, are you looking into the holding tank? You can monitor the level that way. Otherwise, you’ll just have to make your best guess until the level sensors start working.
      There may be a chance that the sensors are ruined and might have to be replaced so keep that in mind.

  2. Need help gray matter tank, when I bought 40 foot trailer Cedar Creek the gauges are really not working too well and it appears that he has the gray tanks always open and draining but the indicators light indicators always stay at 50 or 75%, very very newcomer with RV lifestyle and another question later on mold in the ventilation system or AC unit?
    760-522-9010
    Certainly could use your help and grateful find you on YouTube!! Respectfully submitted
    Wayne B

  3. Super informative. Thank you.
    Rolling in a 2016 Roadtrek Zion so our black tank is only 9.8 gallons but always reads 1/3 full after a tank dump so something is not right. These units are notorious for the gauges not being accurate.
    Looking forward to using your method but dump station fees in these parts are $20 so will have to wait until we go to a spot with hookups and use the Boraxo and Dawn method then. We just won’t get the sloshing around or week-long soaking benefits.
    Will be sharing this article on several RV and Roadtrek Facebook groups I follow. Thanks again.

  4. I bought a 2021 Cyclone 5th wheel on April 3 2021. I live in thr RV part time because of work. I’m here approx 10 to 12 days a month by myself. I noticed the black tank was at about half a month later, so I scheduled a portable toilet company to come out and empty the black tank. Whe I returned a few days later the reading on the tank was the same. Could I have these build-ups you are talking about so soon? The RV is brand new and that was the first tank service. It is a larger tank than I’m use to, so thinking I didn’t add enough water with the chemicals that I should have.
    Thoughts???

    1. Here’s what I’m thinking. You do not want to leave that water in the tank while you’re gone. It’s OK to run 1012 days if you’re if those are concurrent days. 10 to 12 days in a row should be no problem, but you don’t want to leave that water, especially in the black tank. If you’re not going to be there, I would recommend that. If you can work it out that you have that tank pumped.

      Just before you leave and before you leave or before it’s pumped, fill it clear full. Make sure that the tank is full and what’s probably happened is that water is set in the tank, and those sensors are pretty sensitive. That’s kind of funny, but anyway, those sensors will get plugged up with paper.

      Poo, other materials, and what happens is that the sensor doesn’t dry out, so it thinks that the tank is half full. So you want to get that tank empty before you leave that trailer or go back to it. So you want to leave that tank empty if at all possible.

      As usual, 2 to 3 gallons of water start with the Borax in your particular case layer Bill. Because you’ve got the sensors not working, I’d recommend that you use the Borax a half a cup, the dawn of any dish soap and dish detergent. I should say this, so whatever you want to call it, a half a cup of that and run that in the tank for the next three or four tanks. That should help get your sensors cleaned up, so I hope that one helps.

      I also answered this on the live stream Tuesday, June 1st. You can listen to the answer here https://youtu.be/AEUvEPAnpcs?t=446

  5. How do you clean sensors in fresh water tank. The gauge doesn’t go lower then 3/4 full. Would it be the same as the gray tank but flush it out more to clean it.

      1. I have already sanitized the freshwater tank with no improvement to sensor level reading. What else can I try.

            1. It just stopped working. Hum, I suspect you have a wire that came loose or something like that. It’s hard to say over the internet but I would start looking there. I assume your other tanks are working correctly?

    1. I am going to try your Bio-Geo method to clean my black and grey tanks. My 2014 travel trailer sits for weeks/months between trips. We have been using the commercial chemicals. A double dose of the chemical is currently in the black tank with some water.
      1) Can we leave that in the tank while trying the Bio-Geo method?
      2) Going forward should we stop using the chemical treatment all together?

      Thanks for your help

      1. Yes, you can start the Bio-Geo method with the commercial treatments in the tank. You will find that once you start using the Bio-Geo you will not need the commercial treatments. Happy Camping!

    2. A dirty gray or black water tank sensor can produce a smelly problem that makes RV less enjoyable. So, the steps to clean RV holding tank sensors is: First, Drain your gray holding tank and close the tank’s valve. Then, fill your holding tank, add cleaning agent. Open the tank valve and drain the contents, use a pressure washer to spray a powerful water stream through kitchen sink or bathroom drain. Continue dumping holding tank’s content and refilling it with water by opening all RV faucets and showerheads. Ensure to close the valve every time you top up the tank and open the valve when draining.

    3. Just found your videos on the Bio-geo method and had a question. Can I keep the borax with 1/3 water in the black tank for a month between use of my fifth wheel?

      1. AS long as it is with fresh water and not effluent. What I do recommend is that you soak the tank full for a month. Fill the tank full and add 1 cup borax and 1 cup dawn and let it soak for a month. One time should be sufficient and will super clean the tanks. This makes them easier to maintain. But never leave wastewater in the tank for over a month even if you have treated it.

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