Your dashboard indicators are obviously different from my '93 300SD. If there is a malfunction of the glow plug system on my car the glow plug light comes on for a few seconds just after the engine starts, never after it warms up. The check engine light has never come on for any reason in the 8 years that I have owned this car

You may have a problem beyond the sensor. I would be suspicious of the engine wiring harness. You could be given a false warning indication due to wires that are shorting out or grounding intermittently. Since your car seem to start normally I would suspect that the sensor is OK. My car has 66,000 mi. on it and I have noticed cracked insulation both longitudinal and radially. However it has not ben a problem. Mercedes has made a repair harness available just for the engine compartment for most '92 to about '96 models which had the troublesome wiring.