1996 S10 2.2 2WD Truck

Daily driver to and from work 3.5 miles. I do this run 3 times a day. When I drive 30+ mile one way, freeway/highway, on the return run P0137 activates SES. SES clears within 4 - 5 days if driving to and from work 3.5 miles.

New OS. HOS, temperature sensor located under new 180 degree thermostat(original was 180).
Any idea way P0137 trips?

Could be several things.
Weak 02 sensor /02 sensor heater circuit or wiring and plug in or even VCM/PCM computer problem.
Engine running problem can also cause the code.

Needs proper testing and put on a good engine capable scanner that will read out and test 02 sensor.
Any other codes?
MT

No. No other codes. I guess I could switch OS/HOS back to old ones and compare results. That's a cheap test.