o2 sensor code - 89 century

Drive a 89 century, 3.3. I recently got a check engine light while driving home... It stayed on for less than 60 seconds and then went off and I have not seen it again. I checked the stored code and it was a #13 - o2 sensor. I have reason to believe the problem may be with the ECM itself and not the sensor. So my question is: Will the CEL come on everytime the ECM detects an o2 fault? Or will it not come on anymore when it sees a #13 since the code is still stored in the ECM? Thanks.

It will come on again and the ecm is almost guaranteed not to be the problem.

Thanks - some of the other reasons I had suspected the ECM: In the past two weeks, twice has my odometer started to spin backwards while driving. Once I was able to bang the dash to stop it and the other time I had pull over and turn off the car. Not sure if this was a mechanical or electrical problem. Also, my fuel gauge needle seems to be fluctuate more than usual due to acceleration and deceleration. I suppose this could be due to the sending unit though. Any other thoughts? Thanks.

If you can change the speedo issue by hitting the dash top, the pcm is not an issue here. 02 is cheap about 25$ on that car and it is 16 years old!