Hi all,
I've got a 1993 940 wagon that has intermittent failures of the heating system. It has what I believe is called the manual climate control system. Just has a temperature knob on the right side of the control unit. No ability to set as specific temperature.

Three mornings ago, I drove it to work and had no heat. The next two days it worked fine. I haven't been able to catch it in failure mode when I have time to look at it. The heater control valve under the hood is relatively new and the vacuum line seems to be attached well. I think the controls between vents, defrost, etc. are working so that would seem to indicate I'm getting good vacuum to the control unit. On the days the heater works, it seems to operate normally.

Any thoughts on what is going wrong or what to look at? It seems most likely to me that it is an issue in the control unit given that when it works, everything seems to be fine. The FAQ talks about some control board/relay issues but these seem to be more AC related.

Thanks!