UKPassats

Hey guys I’m new to owning a Passat! I have a 2001 B5 1.8t which can’t seem to get up to temperature. I had the expansion tank changed when I bought the car as it was all cruddy and have recently changed both the thermostat and the temperature sensor to try and fix the issue. I have plugged into the car with my OBDII scanner and the fault code P1296 shows. The car takes an age to get up to temperature l, and when it finally does, it drops straight back down as soon as it’s driven on the motorway. It seems to sit ~50 degrees C. Any ideas what the cause to the issue could be? I’m thinking electrical but don’t know where to start looking.

Thanks in advance for any help!!