Rather than replacing a possibly good O2 sensor, put one of these bad boys together.
DIY Air / Fuel Ratio Meter
Get it in closed loop (i.e. > 1800 RPM, steady throttle) and the O2 bar should cycle on / off quickly.
I've been using that and a vacuum gauge to get better FE out of the Lancer. Finding the wires and tapping onto them carefully is the hard part.