Although my year 190E doesnt have the sensor, I have read about them here on the list that they go bad. The sensor is located right there in the coolant expansion tank/resevoir. You can see it at the bottom, on the motor side of the resevoir where it fitts into the res. ( My car has the cutout in the res but no sensor).
Check it out , maybe it is all funked up inside , or something simple like the wire leads are cruddy. I would take it out and clean it up.
I'm not sure if it is related to the check engine light coming on, but it may.
'84 190E 2.3
'83 Porsche 944 (track car)
'84 Porsche 944 (parts car)
85.5 Porsche 944 (sled)