Article 2 of 5 in Series: RV Holding Tank Sensors

Read series from the beginning

Key Points:

• Tank sensors function like a circuit; the rising water level completes the circuit and lights up the level gauge.
• Sensors typically misread because they are covered by something (human waste, toilet paper, grease, oils, etc.).
• When sensors still misread after multiple cleanings, they may just need to be replaced or you can install exterior tank sensors.

Why are my RV holding tank sensors not working? An age-old question. Even many experienced RVers give up on the hope of fixing their holding tank sensors because they are such sensitive, poorly-made devices and can break or malfunction easily. In fact, some people just gauge their habits and take a guess at when their black or gray tank is nearing full because they believe they’ll never be able to rely on their sensors to tell them the truth. We’re here to dispel the myth that you can never trust your sensors. If you are using the right products and practicing the right care habits, that crumbled wall of trust between you and your sensors can be restored!

In this article, we will walk you through how your RV sensors work, why they are malfunctioning, and give you resources on how to fix this problem. Later on in the series, we will walk you through some prevention habits so you don’t have to deal with RV holding tank sensor malfunctions in the future.

How Do RV Tank Sensors Work?

In most cases, an RV tank sensor is mounted on the inside of your tank but some models are mounted on the exterior. Sensors are typically situated in several spots vertically on your tank wall, and the number of sensors varies from model to model.

When the level of water in your tank reaches each sensor, they act as conductors, completing a circuit and lighting up that level reading on the sensor monitor box.

Why Are Your Holding Tank Sensors Not Working?

Sensors malfunction for only two reasons:

1. Something is covering the sensor
2. Sensors are broken (or wiring problems)

Sensors are just circuits that are waiting to be completed by the water level. This means that anything that is covering the sensor (even if it’s not water) can cause the circuit to report tank contents have reached that level when in reality, they have not. This problem is fixed by cleaning the debris off the sensors. This cleaning process varies between black tanks and gray tanks, but if you've applied our recommended cleaning techniques a few times and your sensors still don't read correctly, there is a good chance they are broken and need to be replaced.

Black Tank Sensors

Poop and toilet paper can cake onto the surface of your RV sensors, and droop down into the water below, completing the electrical circuit. This causes your sensors to give false readings that do not accurately reflect the actual water level. Even if you are applying bacteria-based tank treatments to properly liquefy the waste, you can still experience misreading sensors if you are not rinsing your tank effectively after each dump. Holding tanks rely on gravity to drain. Imagine a sink full of dirty dishwater draining; grease and food particles floating on the surface easily get stuck to the sink walls as the waterline slowly recedes. The instinct is to use the spray nozzle in your home kitchen to rinse those clingy particles off so they don’t cause stains or dry out on the sink walls. It’s the same principle with black tanks. Even the most liquefied waste can cling to the walls and sensors as it slowly drains out, so it's important to be sure you're effectively rinsing your black tank each time you dump your black tank.

Toilet paper is one of the usual suspects when it comes to misreading black tank sensors, so be careful to not use too much or opt for a rapidly-dissolving type of toilet paper. You don’t have to use specifically RV toilet paper, but without using a good bacteria-based, waste-digesting treatment product, regular home toilet paper can take longer to break down and get snagged on sensors easily. For more information on whether or not you should use RV toilet paper in your RV, refer to our guide, Is RV Toilet Paper Necessary?

Using bacteria-based products requires an ample amount of water in the tank so the bacteria can survive, flourish, and reach the waste. Extra water also helps soften solid waste and break apart toilet paper, which lessens the chances any of it will stick to the sensors, so be generous with water when flushing.

 

Diagnosing Your Black Tank Sensors. Can They Be Cleaned?

The first thing you need to know is: Can your sensors be fixed by cleaning, or do they need to be fixed - either by replacing the probes, or fixing the wiring?

To determine if your black tank sensors are dirty, or broken, follow these steps:

1. Dump your tank completely, and allow your tank to fully dry
2. When your tank is verifiably empty and dry, check your sensors. If they read "full" you have bad wiring, or bad sensor probes, and they will need to be fixed or replaced. No amount of cleaning them is going to fix your problem.

Let's dig into this a bit deeper. If your tank is dry, there is nothing to complete the electrical circuit because water is the conduit. So, even dirty probes shouldn't light-up in a dry tank. If you see that your sensor panel shows that waste is in the tank, even in a 100% completely dry tank, that is a surefire way to know that you're dealing with a wiring problem or broken probes.

However, if you have misreading sensors that show "empty" after you've allowed your tank to dry out, but then show up as "full" again once you've added some liquid to the black tank, that is a sign that you have dirty probes - and you can restore them by cleaning.

 

Gray/Galley Tank Sensors

Savvy RVers understand that solid food waste shouldn’t be allowed to go down your drain lines and end up in your gray/galley tanks. Even if you go to great lengths to make sure no food waste, grease, or debris ends up inside your gray tank, some will inevitably end up there. When you wash your dishes, pots, or pans (even though you have scraped them clean and maybe even wiped them), some residual food grease will get washed down the drain. Additionally, grease doesn't only come from food waste. Hand soaps, shampoos, body oils, and lotions can all contribute to grease buildup inside of a gray tank. When you're camping with your gray tank valve closed, the same sink analogy we shared earlier also applies to the gray/galley tanks; when dumping, gravity slowly causes the water level to decrease, which allows grease and oils to cling to walls and sensors on the way down.

Hook-up campers who keep their gray valve open don’t need to worry as much about the stealthy bits of grease wreaking havoc on their tank sensors because the waterline never gets high enough for grease and oils to block upper level holding tank sensors. This doesn’t mean, however, that these campers shouldn't deep clean their tanks though. Grease and oil will still create a nasty film on the floor of the tank each time it flows into the sewer, so we recommend deep cleaning your gray tanks at the start and end of every camping seasons, as preventative maintenance (and ever 3-4 months for full timers).

Dry campers/boondockers who must keep their gray tank valves closed can experience a slow buildup of this residual grease on the tank walls and sensors as the waterline rises during use. After multiple dumps where grease and oil stick to sensors as the tank is drained, sensors can begin to read "full" all the time, which is why we always recommend a deep cleaning with Dawn Ultra dish soap the last night of your boondocking trip. 

Hair and hygiene products like soap, shampoo, and toothpaste will end up inside your gray tank as well (which can cause nasty odors); the combination of these items and food grease can make for a powerful concoction of slime that can easily cause tank sensors to misread. But removing this greasy layer is easy and inexpensive and can be done overnight while you sleep! Follow the easy cleaning steps in the next article on How to Clean and Restore RV Holding Tank Sensors.

Note: As we mentioned in this section, how you use your gray water valve can have a huge impact on your sensors, so refer to our Definitive Guide to Using Your RV Waste Water Valves for detailed information on how to use your RV wastewater valves as responsibly as possible.

Combo Tank Sensors

With black/gray combo tanks, the reason for misreading sensors depends on how you’re using your tank. If you are using it as either a black or gray tank only, then the reasons will mirror the ones listed in the above sections. If you are truly using it as a combo tank for both black and gray water, a combination of human waste, toilet paper, grease, and other gray water items could cause misreads. 

How to Clean Your RV Holding Tank Sensors

Now that we’ve covered the reasons for sensor malfunction, and how to diagnose if you can solve your misreading sensors through cleaning, you need actionable steps to perform a sensor cleaning like a champ. Because we believe so strongly that our black tank sensor cleaning product is the best, we use Unique Restore-It as the example product in our instructional steps for cleaning and restoring black tank sensors. We use Dawn Ultra dish soap as the example product for cleaning and restoring gray/galley tank sensors because it is the best grease-eating dish soap on the market and will not disturb the beneficial bacteria colonies that may be in your gray tank.

• Steps for Cleaning Black Tank Sensors
• Steps for Cleaning Gray Tank Sensors

For more information on the best sensor cleaning products and which ones you may want to avoid, refer to our guide on Choosing the Right Products or Methods for Cleaning RV Tank Sensors.

What Can I Do if Several Cleanings Don’t Help?

If you’ve cleaned the sensors several times and they still misread or you routinely struggle with keeping your sensor probes in good operating condition, you might consider installing exterior-mounted sensors, like SeeLevel RV Tank Sensors. Aftermarket sensors like this are a great option if you want more reliable readings on tank levels because it won’t be possible for waste and grease to stick to the sensors themselves and cause misreads. However, if a piece of waste becomes stuck to the inside wall right where the sensor is, it could still misread, so be sure you are practicing good tank care habits even if you use exterior sensors (refer to The Unique Method for more information).

Review

Let’s briefly review what we covered in this article:

• Tank sensors are like circuits that are completed by rising tank levels and are typically placed at four different levels on the tank: bottom, one third up from the bottom, two thirds up from the bottom, and the top.
• Black water sensors misread because they have solid waste or toilet paper covering them, that is draping down into the water below.
• Gray water sensors misread because they have a thick layer of grease and oils caked onto them, which allows for water and debris to create a pathway down into the water below.
• Combo tank sensors misread because of solid waste, toilet paper, grease and/or oils, depending on exactly how you’re using your combo tank.
• Verifiably dry tanks should always read as "empty" because there is no water to complete the circuit. Empty tanks that have sensors that light up indicates a wiring, or sensor probe problem, that needs to be fixed or replaced.
• When sensors still misread after multiple cleanings, they may just need to be replaced or you can install exterior tank sensors.

Now that you have a solid understanding of how the sensors work and what can cause them to misread, the next article will walk you through the steps to clean and restore them to working order.

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