edit: You've got a mk3, my information may be incorrect.
It sounds like the ecu coolant sensor is faulty, it sits on top of the thermostat and should be quite east to change There is a seperate sensor for the gauge in the dash board. This is located on the bottom of the thermostat which is more difficult to change. If you can replace both.
It sounds like the ecu coolant sensor is faulty, it sits on top of the thermostat and should be quite east to change There is a seperate sensor for the gauge in the dash board. This is located on the bottom of the thermostat which is more difficult to change. If you can replace both.