This Baxi thermistor sensor decreases circuit resistance in line with temperature increases. The printed control board (PCB) of a boiler provides a current to the thermistor, reading the relative resistance to determine if a reduction or increase in heat is necessary. This part fits following boilers: MAIN 25 ECO COMBIMAIN 25 ECO ELITE COMBIMAIN 30 ECO COMBIMAIN 30 ECO ELITE COMBIBAXI DUO-TEC 24 COMBI ERP SERIAL... The wiring in a heating appliance connects the electronic control to the numerous sensors, switches and thermostats. These wiring looms allow the pcb to enact safety protocols and control the heating appliance. This product has been discontinued and replaced by product code 666852.