how do you know which o2 sensor has gone for zafira 03 ?

Ask your mechanic to look at the fuel trim on live date to see if the 02 sensor is switching between + and - About 9 % either way. The 02 sensor only measures oxygen in the exhaust you may have an air leak somewhere on the intake. Too much oxygen 02 sensor cannot correct it so it flags a fault code.

Raz
March 2014

If it says bank 1 and bank 2 sensors at fault then my guess it is both sensors required.

cdc
March 2014