So my downstream o2 sensor dies and throws o2 failure code. I replaced the sensor and cleared the CEL. About 100 miles later, CEL again, same code. Cleared it again, haven't driven far/long enough to retest. If it comes back on, what next? Does the ECU in this car need to be hard reset to default by chance?

 

Was the o2 broken?

Make sure you dont have a loose cat internal. It will slide into the o2 and break it.

 

There is no hard reset. only a memory clear.

 

Something is making the ECU throw an O2 code when its not it. Sounds like u might have an exhaust leak post cat but pre O2. How has ur gas mileage been? Also any blinking MIL?

 

+1

I bet the cat is broken and taken out the sensor.

Check out this thread

https://www.s2ki.com/s2000/topic/312...rter-failures/