you don't need a scanner - the ECU is located in the passenger footwell (pull down carpet and you will see it) - it has a red LED that flashes the trouble codes when you have a Check Engine Light - it will flash a number of times, pause, and flash a number of times again, pause, etc. - count the number of flashes between pauses and that will be the code for your problem - you can search for a list of what the codes are - my guess is that you will have a code 1 or code 2 for O2 sensor, based on what you said it is doing - the ECU stores the codes, so the Check Engine Light doesn't need to be on to read the codes - resetting the ECU to clear codes is done by disconnecting the battery or pulling the fuse for the ECU in the underhood fuse box