Yes, the Thermo will only go in one way in fact.
It does not feel like its circulating thru radiator. Overflow was hot, top hose hot, but top of radiator and bottom hose was cool, even when the electric fan kicked in. As I edited above, I broke the top off the red sensor close to the water pump. I think its the fan clutch switch, so the engine fan is not engaging.
Anyone ever put a new thermostat in that was a Dud?

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2009 Mercedes ML320 Bluetec