O2 sensor wiring

[smithy]

ive got a 3 wire O2 sensor which im trying to hook up but would like to know correct wiring before i get too far.
on the sensor there are 2 white and 1 black wires.
i am pretty sure the black would be for earth but does it matter which of the 2 is for ECU reference and heater ( switched power).
or have i got it completely ass about ?

cheers
dean

[smithy]

Got it sorted ;)

for anyone who's interested.....
black- ECU reference wire ( replaces single ECU O2 sensor wire)
white- switched power ( heater)
white 2- earth.