Re: 146 heater problems
Could the heater cable have slipped on it's pulleys?
If it's any help, I had my thermostat replaced yesterday as the engine temp would go to 90, then fall to about 60 without warning then if I was lucky, creep up to 70 then go no further. Also, fuel consumption increased. Thermostat replaced (£29.50 for Quinton and Hazel part from a local Motor Factors and £20 to fit). The heater temp is now lovely and warm and fuel consumption has dropped again.
It could be that your temp sensor is also broken hence saying it's stable; but I am more prone to think it's the heater cable. Swine of a job as the majority of the console has to be removed to get at it.
Hope this helps?