Dear RV Doctor,

I think there is something wrong with the monitor panel in my RV. I just dumped both holding tanks and still the 1/4, the 1/2, and the Full lights are lit for the black water holding tank. On the gray tank, it still shows 1/4 full. The water tank, however, registers fine. I know the holding tanks are completely empty….I just emptied them! Do I need to replace the monitor panel?


Oh, if only I had a quarter for every question like the one above that has come into the RV Doctor Column over the last three plus decades! Not a week goes by that I don’t receive a similar email. Gary, are you saying this problem has been around for over thirty years? Sadly, yes! The good news is that there is now a very real and very reliable solution. But first, here’s a little explanation of how those in-tank monitoring probes become fouled, resulting in those erratic indications at the monitor panel.

In-the-tank probes literally extend through the sidewall of the holding tank and well into the tank. Most waste tank probes extend horizontally into each tank (some fresh water tank probes were vertically installed). As the tank fills with waste, the liquid portion of the contents would make contact with each probe positioned at various levels up the wall of the tank. Some tanks have probes positioned at the 1/4, 1/2, 3/4 and Full levels. Other tanks are divided by thirds; 1/3, 2/3 and Full.

bunzer1 In addition to the three or four probes at individual levels, there are also one or two additional probes mounted alongside the column of level probes. These “common” probes ultimately connect to one specific circuit at the monitor panel for that tank. Here’s an inside view of a tank with four level probes and two common probes.

In a nutshell, as the liquid rises inside the tank, (remember, water is conductive), it makes contact with at least one of the common probes and each successive level probe, thereby completing that circuit and lighting that lamp or LED on the monitor panel inside the RV.

The typical probe or “well nut” is nothing more than a stainless steel machine screw threaded through an embedded brass nut inside a rubber grommet. Tightening the machine screw effectively creates a leak-proof seal and attaches the monitor panel wire to the probe. well nut
Another type of well nut has the screw reversed. An installation nut, plus a nut to secure the monitor panel wire, is located on the outside of the tank while the screw “head” actually becomes the portion that contacts the tank contents on the inside. screw reversed
bunzer4 As you can see in this photo, the screw portion of typical well nuts extend quite a way into the tank, creating a very nice set of “hooks” – the perfect hook, in fact, for waste and sludge to adhere to and to ultimately bleed conductance to other probes. The metallic portion of the machine screw is actually fairly close to the tank wall. And this is the very reason the monitor panel will eventually create false indications. Tissue, waste and sludge will build up over time, creating in essence, a short circuit between the probes because of the low resistance between the probes, the tank wall and each other. Moist tissue draped over the probes will most certainly pass enough conductance to “trick” the monitor panel into thinking that tank still contains waste.
So what is to be done? Well, for years we’ve recommended holding tank additives, thorough rinsings, even bags of ice cubes (in the black holding tank) to help eliminate this probe-fouling problem, but yet the dilemma persists.  Enter Horst Miracle Probes.  Produced by Horst Dynamics, Inc., the Miracle Probes are replacement probes for the common well nut or for any existing in-tank monitor probe, including those that have been installed by a spin-welding process during the tank manufacture. bunzer4

One cursory glance at these revolutionary probes and it should be evident that these high resistance replacement probes are certainly very different. A closer look at the design will reveal the intent behind that design. Simply put, the installation of the Miracle Probes will likely eliminate false monitor panel readings….forever! In fact, because of their design, it’s almost impossible for these probes to foul under normal use. Personally, I doubt it’s possible to render these probes fouled, even intentionally.

bunzer6 Take a look at the Miracle Probe designed for the black waste holding tank. Notice the opaque protective Teflon® tubing over the tiny metal portion of the probe? Teflon®, by nature, is non-wetting and self-cleaning. Also notice the white plastic section over the total length of the probe and beyond. This “roof” section literally keeps waste, sludge and tissue from contacting the electrical portion of the probe.
Because of the protective roof, the black tank Miracle Probe must be installed to the correct orientation to ensure the roof section remains situated above the probe. This is effectuated by making sure the non-milled portion of the exterior threaded stud, shown here, is positioned on top while tightening the mounting nut. Notice it even looks a little like a curved roof overhang. There is no special orientation required for the gray tank probes. bunzer7
bunzer8 The gray tank Miracle Probe, pictured here, is also constructed with an opaque Teflon® tubing over the straight metallic section, leaving only the spiral conductive head of the probe exposed and well away from the tank wall. This greater distance from the tank wall coupled with the small diameter of the probe itself minimizes or eliminates the possibility of a false positive or current leakage from one probe to another.

The internal construction, that portion you cannot see, is impressive too. The conductive head is internally connected to only the brass stud on the outside of the tank. Everything else is totally insulated from the current carrying components, including the tank wall. The monitor panel can now receive a very precise signal. Imagine that; an accurate monitor panel!

Like the common well nut, the Horst Miracle Probes are inserted through a drilled, 3/8-inch hole in the tank wall. This simplifies the installation such that any existing well nut can be easily replaced. Just be sure the holding tanks are empty first! Then simply reconnect the original monitor panel wiring. No new wiring is required.

On some tanks, the existing probes may be spin-welded into the side of the tank; different from the standard well nuts explained above. For fouled probes that are atypical, simply drill a new 3/8-inch hole at the same level as the fouled probe and install the Horst Miracle Probe. Remember, if it’s on the solid waste holding tank, be sure the “roof” section remains on top!

So if it’s this simple, Doc, why wasn’t this invented long ago? Well, I’ve learned not to focus so much on the “why” questions, rather, I like to explore the “how” answers. “Why” it took so long, I’ll never know, but I now know “how” to eliminate the problem of unpredictable and unreliable monitor panel readings caused by fouled in-tank probes. And so do you!

If you are what I call a “serious” RVer, (you know who you are), you owe it to yourself to investigate this new “miracle” for the RV waste system. If you’ve ever experienced the frustration of inaccurate monitor panel indications, you won’t need much convincing to make the switch to the Horst Miracle Probes.

And remember, RVing is more than a hobby, it’s a lifestyle!

(Please feel free to comment, however, please also note that due to the volume of communications I receive from multiple channels I cannot guarantee a personal response in every instance. However, questions of an overall general interest may be considered and published in an upcoming RV Doctor column.)

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  1. RV Steve

    Folks, I would like to pass on an old school trick that has worked for me an many others regarding sensor misreadings. ICE ICE AND A LITTLE MORE ICE! First flush your Black Tank then flush the Gray Tank to clean the hose. Put everything away and before you say “Hit The Road Jack” feed your toilet 4 to 5 bags of ice. As your drive down the road… stop, go, left, right… that ice will slury around your Black Tank and hit the Sensors thus cleaning them. Add a little liquid lysol from the dollar store and you can deoderize your tank at the same time. I am sure there are many tricks to the trade and God knows I would have to read many books to keep up with Mr. Gary Bunzer but this method has worked for me and for the folks I suggested it to in the past.

  2. Eldon Ashbaugh

    On the subject of the tank probes, I was having the usual trouble so I bought the tornado rinser and attached it to my tank. When I ues it had a lot of bad stuff that was causing my tank to plug up just under the toilet, probably been there for years. When I cut the hole for the rinser, I had to relocate a probe. Trying to remove the probe and it fell into the tank so I flushed it out. It would not go in properly, soooooooo I went to the hardware store and found stainless steel machine screws that were the length of the thickness of the tank wall plus the thickness of the wire connector. I drilled holes in the tank and screwed the screws in so they were either fluse or slightly recessed in the side of the tank. You might have to grind the end of them so they don’t extend into the tank. I did not use any sealer since the screws went in very tight. I have not had any problems with leakage as of yet but I have only used it a couple of times. I figured the probes not sticking into the tank their would not be anything to get hung up on. It will probably be next spring before I have a chance to give them a real hard test.

  3. Dalton Tamney

    As was suggested I tried to find where the probes are on my holding tank in my motorhome. No luck. The entire holding tank is apparently enclosed inside the rig with no obvioust access to the probes. I guess I am stuck with the ones I have even though they are useless. Manufacturers should be looking to install the best probes available instead of those that do nothing but cause aggravation. I think Horst Miracle Probes will find a smaller market than they originally anticipated since, I suspect, the aftermarket sales will be limited to the relatively small number of rigs for which these probes are accessible. Such a shame.

  4. Thomas Becher

    Always a day late and a dollar short. I just installed a Garanett SeaLevel guage. But I know it will never short out or fail because of junk in the tank. It works on capacitance thru the walls of the tank. The new product sounds good and I would have tried it.

  5. Harf Overton

    We have a Holiday Rambler (07 Endeavor) with a Techma toilet with outside sensors. Sometimes they work and sometimes they don’t. We have tried everything, I think. Very annoying. Any ideas?

  6. Phil Schoner

    I have a 2007 Monaco Dynasty with an Alladin system. It shows tank levels in 10% increments. I think it works on pressure readings, not electrical probes. I say this because when we travel to different elevations, the readings will be obviously in error for a day or so until the system adjusts to the new atmospheric pressure.



    I have the same problem with my holding tanks. but a little different, mine will show empty but will show different readings when not using the rv ; any comments. THANKS FLOYD

  8. BikerRon70

    I guess someone always has to be negative so it is me.

    I bought these Horst probes, for both tanks, and installed them on our 2007 Coachmen Class C. I bought the appropriate probes for the gray and black water tanks. I had to drill new holes in the tanks, install the new probes, and switch the wires. I was concerned about leakage issues and if the expandable rubber doughnut would really work properly. They do seem to be holding and not leaking.

    We are campground hosts for the month of October in a Wisconsin State Park, with electricity and water hookup, but no sewer hookup. Our gray water tank seems to fill up quickly so last week I drove over to the dump station to dump the tanks. After dumping and using a tornado tank washer in the black tank, I was still getting a reading of 1/2 full on the monitor panel with the 3/4 light just starting to glow. After a week, today the monitor panel shows that the black water tank is full. I know it is not full. We haven’t been using the toilet that much. When in doubt use the old “look in the tank through the flushing toilet with a flashlight trick” to see if the black water tank is really full.

    The gray water tank does fill faster. I tell my wife that we need to eat out more often. We don’t shower in the MH unless we have a sewer hookup. The probes in the gray tank worked fine showing empty after emptying and now are at 3/4.

  9. catchesthewind

    Gary, This is one of the best written and easy to understand articles I have ever read. I will print this out and use it for reference and replace my old sensor probes. Thank You very much.

  10. catchesthewind

    Dalton, Start from where you hook up your drain hose and follow the drain pipe back to the tank. If you dont see the sensor location right away check if your tank is encased in insulation. If so try to find where the wires exit the tank and go from there. Good luck!!!!!!!!!!!!!!!!!!!!!!!!!

  11. Tireman9

    I also have problems with getting a correct level reading but I am an Engineer and have tested my system and do not believe it is dirty senders.
    My black tank is a 27 gal unit. The first year I had our Coachmen I noted the tank seemed to fill very quickly but I dutifully dumped my tank frequently.
    Earlier this summer I was considering getting the exterior sensors after helping a friend with his tank sensors but first I decided to confirm the cause for my problem.
    I dumped my tank. Then I used the sprayer wand you can put down the toilet to be sure stuff was cleaned off the tank sides. I then filled the tank with just water from a hose TWO times.
    I now felt the tank was clean.
    I then added measured amounts of water in 3 gal increments and checked the reading. At empty and w/3 gal in the tank the sensor panes read empty.
    I continued to add 3 gal at a time and checked the tank level light panel.
    after adding a toal of 6 gal the indicator registered 1/3 full which is a little high. The real problem became aparent when the panel read 2/3 full with 9 gal and FULL after only 15 gal.
    Clearly this isn’t a dirty sensor problem. Just a failure of the manufacturer to place the sensors at the correct locations.

    Now I use a flashlight, look down the toilet and know when the tank is getting full.

    So much for Quality work by RV manufacturers.

  12. Bluebird Bob

    I changed to MicroPulse.
    These probes CAN’T foul as they go by pressure only. Doesn’t matter what’s floating by.
    Had them in my older Bluebird for 4 years now and TOTALLY accurate.
    Monitor costs $125 and each probe is $25.
    Best product we have put on our Bird!!

  13. Darthvagrant

    The black water probes look like a good idea and design. My question would be “how do you know if the Teflon ‘shield’ hasn”t turned during installation. When the nut is tightened to expand the rubber collar, perhaps the shield has become a scoop? Is there any method of determining the position from the outside? Viewing from inside the tank would normally be impossible. Uh-oh! Is that a flat area on the end of stud to determine? And if so, one would assume the threaded stud is secured to the shield?
    I think I’m ready for a set!

  14. Harris Schultz

    sounds like the Horst miracle probe is a “slam dunk” fix. But I’m aware from reading the mags that there are other ways of measuring the content levels of holding tanks. Our unit has the 1/2, 2/3 etc scale, and when the 1/3 light fails to light my traveling companion says our x tank is empty. At 60 gals, that means we may have near 30 gals, enuf to get us thru the day/night. Aren’t there devices that show the percentage of full or empty?

  15. Dalton Tamney

    Halleluia! Finally someone has done something about these previously absolutely useless blackwater probes. My only problem is finding the location of the old probles on my holding tanks. I have a 38′ Alfa Class A. If anyone knows how to get access to these probes on the black and grey tanks I will be forever in your debt.