Never seen the dash display on your model, but I would guess that would be the coolant temp sensor...

...does the owner's manual say anything about this?

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car