First thing to check then is amps through the heater and depending on the results from that the power supply to the unit then.
Dont need a wiring diagram for basics like this, heater circuit on old sensors is one of the white wires, on newer 5/6 wire sensors just look at the wire thickness from the car to the 02 multi-plug. There will be two wires thicker than the others, these are the heater circuit wires (higher current).
Amps clamp or at least check the power,then the control side.
Hth Sean