I said rest button, not reset button. More info is available at this thread.
CLK 430 Problem
Driving the car with a lower than normal coolant temp will give you worse gas mileage and over an extended period of time can do more wear to the engine if it is not reaching full temp. I do not think these cars should really be below 80 at all with a properly functioning thermostat. Definitely replace the thermostat, it is an easy DIY job and is only about a $10 part.
[Edited by Ali Al-Chalabi on 02-15-2001 at 01:18 PM]