If a toilet overflows spontaneously without being clogged, the issue is almost always caused by a malfunctioning fill valve or float mechanism that fails to shut off the water supply when the tank reaches the proper level. This causes water to pour continuously into the overflow tube, filling the tank and bowl faster than the siphon jet can drain it, resulting in an overflow. Callout type="warning" title="Hygiene Warning": Shut off the water supply valve immediately to stop the overflow. Verification: Check that the fill valve float stops moving upward and cuts off water flow at least an inch below the top of the overflow tube.
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