Oil Tank Watchman Problem

Hi Everyone
Just wondering if anyone has a solution to the Watchman issue I have with my tank. Its been in place for 5 years and working fine. Recently it has shown a fault of the 5th bar flashing next to an exclamation mark in a triangle.
Looking at the fault codes it seems this could be "oil sweat" or debris in the tank. I`ve cleaned out the sweat and there is no debris in the tank that I can feel. I replaced the battery too as it hadn`t been done since new but the fault persists. I`ve linked the sender and transmitter again as well but just goes straight back to the fault.
I have a low profile tank and buttons 6 & 8 are raised on the receiver, again its been working fine for 5 years but tried 6 & 7 as was suggested but still the same.
I`m pretty much out of ideas now other than buying a new set but surely it should last longer than 5 years ?
Thanks for your time, any further advice greatefully received, cheers