Occupant detection system and method

An occupant detection system (30) and method (300) that uses an isolation assembly (40) to isolate electrically a power supply (32) providing power to a heater element (14) for heating a vehicle seat from a sensing circuit (60) that uses the heater element (14) as an electrode for electric field (34) or capacitance type occupant detection. The isolation assembly (40) includes a common mode choke that by itself provides sufficient isolation when the sensing circuit (60) excitation signal frequency is high enough, and provides switches that infrequently interrupt electrical power to the heater element (14) so a humidity level can be determined using a relatively low excitation signal frequency. Knowing the humidity level is desirable to adjust a threshold (500) use to determine if the sensing circuit (60) indicates that the seat is empty, or is occupied.