A couple of years ago the VW (2006 Jetta) quit warming up.
No heat, no defrost, temperature gauge reads cold!
I changed thermostat and tested both 'stats at the same time. Both opened right at 195 °F.
TDI Forum has not been much help. Only "cure" that they recommended was to stuff the grill with foam pipe insulation or otherwise block the radiator. This helps but is definitely NOT a cure.
The engine also runs cold in the summer - it runs fine without blockage, but running cold is not good for any engine!
Any ideas? Thank heaven for seat heaters!