* As you probqbly know, temperature sensors are thermal resistors. The resistance changes as a function of temperature. So "checking" one means measuring the resistance through the full temperature range. If you only check the sensor with the engine at operating temperature, and the sensor is broken/stuck in that reading, you'll get the appropriate resistance and still have the engine problem at colder temps.
* Thoroughly checking a temp sensor (not just taking a spot reading) is time consuming and a logistical hassle. Temp sensors aren't that expensive and are usually far easier to replace than to thoroughly check. If the symptoms point to a possible faulty temp sensor I just replace them. In fact, on higher mileage engines I routinely replace temp sensors as part of preventive maintenance (in the same way that I would replace plug wires). Realize that you've got maybe a $40 sensor providing information to manage several thousand dollars worth of controllers and actuators.
* Also, a new temp sensor can get rid of a variety of intermittant, mysterious and ideosyncratic symptoms.

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ASE Master Tech
Quality Enterprises
Mercedes Svc. & Restoration
Sheridan, IN
317.769.3536