Could be the sensor that is bad. Unplug the wire that is going to the sensor on the bottom of the antifreze tank and clean the electrical contacts. Also how many miles on the car. Head gasket was done on it?
If not, there is a problem.

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New Addition to the family
2013 Passat TDI 5Miles
2004 E500 4MATIC Wagon 95k
1996 E300 Diesel 173k Sold
1993 300E 2.8 230k Sold
1994 E320 White Wagon 373K Sold
Still dreaming about ....500E


Radu Negru