I have 2012, R350, with about 148,000 miles that has set a PO128 code. Low engine operating temp. The temp. gauge is indicating 90, degrees Celsius. After driving about 5 miles I checked the engine temp. With an inferred heat gun. The temps. We’re around 170, at the thermostat housing. So the questions are, what temps. should I see at the thermostat housing for proper engine operation? Do I need to replace the thermostat? Or has the temp. sensor gone bad.
Looking for guidance.