One third means one third doesn’t it? Any technician can tell you that where the sensor probes were designed to be and where they actually are can be two totally different subjects. The most common sensor probe is a steel insert drilled through and sealed to the wall of the tank. On the inside it is just a little silver nob. On the outside it looks like a screw head with a color-coded wire attached to it. The bottom probe is a ground probe attached to the frame. The other wires carry a small electrical charge that, when connected by water will light up the sensor panel in the coach. Electronic sensors are a different subject and not on the more basic RVs. These sensors work perfectly for years and years if installed correctly. Usually the lights are on because the tank is dirty, not because the sensor is misreading. The sensor is telling the truth but up until TankTechs discovered a way to hydro-jet clean sensors in the tank and then developed TankTechsRx to keep them clean they got an unfair bad rap for never working. Sensors are only as accurate as the technician who installed them at the factory was. We have seen some of the strangest sensor probe placements you can imagine. Theoretically a 1/3 sensor should be 1/3 from the bottom, the 2/3 and equal distance up from there and the full sensor not quite to the top leaving a little spare storage room. Name the pattern and we have probably seen it: side by side, triangle, box, vertical and random. It is easy to visually inspect if you can see the side of the tank. Badly placed sensors can be replaced simply by disconnecting the wires and screwing in a new sensor in the correct position. Then just reconnect the color coded wire. (There is no guarantee they used the right color wire. They used what they had closest to them at the time). The old sensors will do no harm if you just abandon them. Using TankTechsRx regularly will keep them telling the truth when the tank is clean.