If you get no heat, first just rebuild the monovalve. I've had the "no heat situation" twice in my SDL (126,000 mi.) and each time it was the monovalve. It's easy to work on and there is even a thread that has pictures to follow. There may be another problem, but the monovalve has a reputation for going bad. If you do decide to get an control head, E-mail me and I'll Give you the Web Site where you can get a rebuilt one for, I think, $155. Good Luck.
