One additional question: does the heater work normally? If not, the engine is indeed not getting warm and the thermostat is the first place to look. If the heater works normally, then the engine is probably OK and you should check the temp gauge and its sender.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970