Your read out panel may be for a TankWatch 4, but, based on your photo, that's not what you have. This is what TW 4 sensors look like.
If I were you I'd replace the whole thing with a SCAD Tank Monitor...top rated, reasonably priced (available from a number of retailers including Amazon), the sensors go on the outside of the tank, so can never become clogged as internal sensor do. The TW 4 wiring--at least the ORIGINAL wiring (who knows what you have now)--should work with it. Scad Tank Monitors