no o2 sensor or failing one you are gonna be running pig rich which could/will foul out plugs and create a no start situation. As far as resetting the ecu, if you disconnect the negative battery terminal for 30 minutes it will drain all the backup power to the ecu, you usually do this also when servicing airbags to be sure that the charge is completely gone. It is a hard reset and should clear anything found in the ECU which may inhibit the car from starting.