We just recently learned about the Geo Method and that is the only way we are going from now on. We have a new camper and had some issues with false readings on black and gray tanks. We spent several "sessions" cleaning out the black tank and finally got it to read Empty. The last session we used a bottle of Super Clean, as advised by someone. It did get a bit more stuff out of the black tank, but the little bit we put in the gray tank just to be sure it was cleaned out, well, the suds from the Super Clean caused a false reading on the gray tank causing us to rinse some more and then we were good to go. So what I am sayin' here is even a soap bubble can mess with your sensors. We will skip the Super Clean next time as it is too sudsy. Do the Geo Method, let it soak, slosh it around and rinse, rinse, rinse, rinse and rinse some more. However, since yours is unused, that is strange. Those sensors are very sensitive. If it is possible, take it back to dealer and they can get it working.