to check O2 to see if it's running closed loop and responding fast enough, warm up the engine fully then there's 4 ways to check the sensor:
Oscilliscope: a normal digital multimeter ain't responsive enough to show the O2 voltage oscilate twice a sec without a laggy blur of numbers.
preferably tap an oscilliscope into the signal wire and view the duty cycle of the sensor output at 2k rpm, should go up/down rich/lean bout twice/sec.
any slower and the sensor is getting worn.
Shorting diagnostic port: without any consult cable or oscilliscope, on most later generation micras you can set the ECU into diagnostic mode with a paperclip trick & looking at the engine light.
turn IGN on, using a paperclip you short the "Ign" & "Check" pin on the diagnostic plug by the drivers fuse box for 2sec then remove. pay careful attention to the pattern of the pin holes and dividing slots.
on pre-facelifts it's the lower left pins
View attachment 44197
on face-lifts it's upside down, the upper right pins
View attachment 44198
while engine's not running yet it's in diagnostic mode 1 and simply flashes any stored ECU codes via the engine light, long flashes = first number, short flashes = 2nd number.
to check the O2 sensor in diagnostic mode 2, start the warm engine and hold 2k. the engine light now represents the sensor, ON = lean, OFF = rich, should blink twice per sec.
laptop consult program: with a consult cable and free ECU diagnostic software or nissan datascan, switch IGN on, connect to the ECU, view sensor readings, warm up & hold 2k rpm and the primary O2 sensor should go rich/lean rapidly twice per sec
consult diagnostic device: same as the laptop but with a standalone device
To check ECU coolant sensor to see if it's reading too cold (high Ohm or disconnected) making it run a permanently cold rich mixture: remove the 2-pin temperature sensor, connect to a multimeter, dip in boiled water and log the resistance against degree C. Should match this
https://www.micra.org.uk/threads/pollymobiles-rebuild.35251/page-119#post-704079
