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Holding tank gauges are notoriously inaccurate and prone to sticking in older boats. especially if the head is used for more than just liquids and especially if toilet paper is used, even if it is marine toilet paper. I would recommend putting some Rid-X in the tank, leave it for a few days, then take the boat out to shake up your holding tank and then pump out and rinse 2-3 full holding tanks worth of water, preferably hot if you can, for best results. If this doesn't loosen up the holding tank sensor and make it work again, then it is probably in need of replacement.

Jim G.