This sounds very similar to the problem I was having with my 98 SL2. I replaced the thermostat because my car wasn't getting warm and the ECTS. I also had to replace the O2 sensor after driving the car for about 60 miles. My car had been running rich for about a year or more and wiped the O2 sensor. It appears that my car is now fixed. Here is a link to my post.

https://www.saturnforum.com/forum/saturn-s-series-sedan-27/new-forum-questions-about-sl2-6564/

Well tonight after work the car would not start. It was so flooded that it could not start.

So I pulled the injector fuse and it immediately started until it ran out of fuel in the cylinders. I then reinstalled the injector fuse and it started up although it still took several seconds to clean out the cylinders of the fuel.

I was able to get an extra $55 bux on a side job today so I have the cash for the parts. I will be installing a new ETCS and connector along with a new thermostat and coolant flush tomorrow.

OK, i installed the Genuine Saturn ECTS and Connector. The sensor was plastic tipped and all cracked up. The connector was corroded as well.

I went ahead and flushed the coolant system and installed a new Thermostat and gasket to fix the stuck open OEM thermostat.

The car started nicely and once I burped the coolant system and verified the upper and lower hoses were getting warm I drove the car for about 15 miles. Sure enough the SERVICE ENGINE SOON light came on which you had warned me about.

So my question is should I just replace the Oxygen Sensor or should I see if the local Advance Auto Parts store can pull the trouble code? Or can I pull the trouble codes somehow?

If the Oxygen sensor is bad can I use a replacement from Advance Auto or do I need a Genuine Saturn replacement part as well?

Swing by an AutoZone, O'Reiley's, etc and let them run a scan. Post the NUMBERS here, and we'll go from there ....

Since I was out of cash to spend I disconnected the battery, and removed the 02 Sensor. It had some carbon on it and I wire brushed it off, applied anti- seize on the threads and reinstalled it.

I have driven the car 40 miles since I did this and the Service Engine Soon light has not came back on.

So far the car seems to be starting well and idling noticeably smoother and right about 900 RPM.

I will follow up with an update.

I want everyone who posted so far offering assistance that I appreciate your time and knowledge very much. I am a member of several forums and this has been the most helpful and welcoming forum I have ever been a part of.

If you do end up needing a new front 02 sensor, go with a Denso replacement, not a Bosch. (personal experience)

OK, I will make a note of that.

Thanks for posting back and letting us know what you've done. Sometimes people get solutions and just disappear.

Yep, I know and then when someone else has the same problem they are left wondering what the actual solution was.

I will be sure and keep this thread updated so future readers know the start to finish.

Day 5 since I installed the new Sensor,Thermostat, Flushed Coolant, and Wire Brushed 02 Sensor.

Car is starting nicely, Idle is noticeably improved and smoother, Gas mileage seems better although I have not confirmed exact MPG.

I was getting about 39 MPG on Road Trips prior to the problems. However the Thermostat has always been stuck open since I have had the car, and was running on all original tune up until 100k miles.

I would suspect I should gain several MPG now with the fresh Fuel Filter,Spark Plugs, Thermostat, Coolant, ECTS, Air Filter, Clean 02 Sensor, and Clean Throttle Body.