Never try to clean the terminals with WD40. I did it and O2 and its wiring got short :(. And another bad thing, ignition switch terminal melted due to this short circuit, but the ignition fuses remained intact happily :-#.
Alhamdulillah, the ECU survived it. Now O2 is disconnected. And i know it is running very rich at the moment.
Before installing new O2, I'll also need to inspect the O2 wires leading to ECU for any short circuit as well.