Test
Look in search here under holding tank and who makes your's
how to test it with meter.
My old 290 was hard to remove and did get it out after removing screws and sealer but still did not work good.
My H-36 has stainless sender and have cleaned it and dealer replaced when it was new and still show's un accurate.
have heard good things about a new one that the sender sticks on sides of tank
and need new gauge $$$$.
I just look in one of my rear stern compartments and can see holding tank level
and not spending any more $$$$.
Nick