W124/M102 230TE Auto Trans - cabin heating ON constantly

[Mercaholic]

Hi Guys - I have a unit without air-con, it has two controls on the dash - one for each side of the cabin - blue at one end for cold, grading to solid red the other, with the TEMP markings 18, 20, 22, 24, 26 on each. Usually when these are set on a temp marking, you can hear a valve kicking in/out. Now there is no sound, even if set on MIN the heat is on MAX.

Could this be the sensor in the sunroof complex, or is the valve that's died??

Thanks