goFirst of all your car does not have a monovalve. This was the type heater valve used on the 123 and 126 cars of the seventies and eighties. Yours has a valve that is not rebuildable.
Secondly, since it is acting eratically, you need to see that the fan that draws air over the temp sensor is working. I'm not intimately familiar with the 202 cars, but I expect that there is a small grill near the rear view mirror. With the key on, see if it will draw smoke. A cigarette works good for this. If you don't smoke find some other way to put smoke there.
If it does not draw smoke, you need to find the fan and check and possibly replace it.