RV Blackwater Tank Sensor Cleaning: Someone’s Gotta Do It!

Last week, we discussed the blackwater holding tank, which can stash both solid and liquid waste. It’s not a pretty thing to see, that’s for sure, but the stink and gunk that can build up if you don’t filter this tank out can be much more gruesome.

How do you know when it’s time to give your blackwater tank a cleaning? It turns out this tank has a series of sensors within it that’s supposed to let you know when to take care of that less-than-desirable task. Sometimes though, these sensors don’t work as they should, causing you extra work, since you’re now scouring a semi-clean blackwater tank.

In this article, we’ll discuss in-depth about what these sensors look like, why they may malfunction, and how to keep them clean.

What Are Your Blackwater Tank Sensors and What Do They Look Like?

As mentioned in the intro, your blackwater tank sensors are built within the tank to indicate when it’s time to clean it. This saves you the time from having to open this unappealing tank and guessing whether it’s time to clean it yourself.

There will also be an external monitor panel that is installed into the wall of your RV that you can use to gauge cleanliness. This panel has indicator lights that let you know when the waste within the tank is filling up and when it should be emptied.

The sensors, which are probes with screws, are stacked vertically from the top of the blackwater tank to the bottom. Each sensor goes off when waste accumulates to that level. If there’s a little bit of waste, then the bottom sensor will go off. As waste builds up, then the middle sensors will go off. As waste gets to the point of needing to be emptied, then the top sensor will go off.

Supposedly, it’s as simple as that, yet many RV owners can tell you their own horror stories with their blackwater tank sensors not quite working the way they should…

Why Do They Malfunction?

There are plenty of reasons your blackwater tank sensors may malfunction. If they were inexpensive from the get-go, then they probably won’t work well for long. If too much gunk saturates over the screws, then the sensors may stop giving readings or may generate inaccurate readings about waste levels.

The type of waste in the tank can also lead to malfunctioning tank sensors. When we say waste, we’re not just talking about bathroom waste and water, although that’s where this goes, too. We’re also including body oils, toothpaste, shampoo, soap, and other bathroom messes. Still, the worst of the worst offenders is toilet paper.

Think about it. How many times has your toilet been backed up by too much toilet paper? Those wet globs don’t break down the same way the other waste mentioned above does. A toilet paper glob can block any of the sensors and thus alter your reading. Sometimes these toilet paper pieces hook onto the screws and make their new home there, meaning that each time you check your external monitor, you’ll get an improper reading.

Even sitting in all that liquid and other waste, toilet paper doesn’t break down for a quite a long time, which is problematic.

How to Keep These Sensors Clean

Now that you’re aware of some of the problems that may afflict your RV’s blackwater tank sensors, how do you go about keeping them clean? That’s a good question.

There are plenty of tactics and methods that RV owners like yourself will recommend. One of these is known as the GEO Method. This has been a favorite for more than 30 years, so you know it’s at least worth trying once. Essentially, you’re mixing dish detergent (Dawn is recommended, but any brand works, even laundry detergent) with water softener. Calgon is a popular brand for the job, but again, get whichever brand you like.

The reason you’re using dish detergent is to reduce grime. The water softener will eliminate hard mineral deposits, scum, and water surface tension. You fill a cup of each substance and then pour them both into the blackwater tank. These will gradually do their thing, and you should get a more accurate sensor reading from now on.

You also have other options, such as adding a large ice cube bag into your tank. Supposedly, the ice breaks down and cleans the tank without any elbow grease on your part. Some RV owners love this method while others aren’t as crazy about it, but it’s there.

If your tank didn’t come with its own back-flush system, you can always order an after-market one and hook it up yourself. This keeps the water moving and flushing through the tank so that even if toilet paper gets stuck on a sensor, it won’t stay long.

You can also use a standard garden hose and a spray wand. You want to turn the hose to the highest pressure possible. Then, with the water from the hose cleaning out the tank, use your spray wand to dissipate stubborn toilet paper.

Lastly, you can always switch toilet paper brands to one made for RVs. This Thetford toilet paper is single-ply, but it has four stars on Amazon and costs less than $8 for four rolls.

No matter which method you prefer, you should always let your tank fill up two-thirds of the way and then dump it. Why? Dumping too soon can affect the pressure and thus lead to what’s known as tank pyramids. These are an accumulation of dried-up waste and other gunk. Trust us when we say you don’t want to deal with this issue if you don’t have to!

So there you have it. Although this isn’t a fun job to do, it’s a necessary one.


Blackwater tanks have small sensors running down the length of them. These sensors indicate when it’s time to dump out the tank’s less-than-favorable contents.

Things like toilet paper residue and other gunk can mess up the sensor readings, giving you inaccurate information. By following the cleaning tips above, such as using dish detergent and water softener as part of the GEO Method or even grabbing an old garden hose and a spray wand, you can keep the blackwater tank sensors clean.


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