A system for preventing overflow of a toilet includes a capacitance sensor arrangement as in combination with certain features (e.g., flap valve disablement). The system can detect an overflow flush condition of a toilet by sensing the value of a parameter of one or more normal flush cycles of the toilet and establishing a normal range for the value of the parameter using the sensed value. The system can also store the normal range in a memory for comparison to a value of the parameter during subsequent flush cycles. The system can perform sensing over a predetermined timeframe. The system can include the time frame being sufficient to include the entire push cycle of the toilet.