My check engine light has been on, and I just found the fault codes (thanks to the help of many of you on this site), and it is most likely that I need to change the sensor(s). To my understanding there is an upstream and downstream sensor.

My car runs well, no hesitations, no startup issues, and no noticable loss of gas mileage. Do I need to change both sensors, or should I just do it anyway (ie. is one more likely to be the culprit)?

I am inclined, with 98k on the car, to just replace them both, is this a smart move?

Finally, I have been looking for sensors and noticed that I can get universal sensors for up to less than 2/3 the price of OEM. The universals require splicing, etc... Should I go with OEM fits, or will the universals do the same job? Also, I noticed brands like Bosch have both universal and OEM, which makes me wonder if the only difference is the "exact fitting" vs. splicing fits. So, if I get a universal, will it do the same job as the OEM, as long as it's installed correctly?

Thank you.