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.

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Patrick Leber
'84 190E 2.3
'83 Porsche 944 (track car)
'84 Porsche 944 (parts car)
85.5 Porsche 944 (sled)