Start with the basics.

Open up the monovalve and check the condition of the diaphragm. 90% of the problem with "no heat" is caused by a tear in the monovalve diaphragm.

If the diaphragm is torn, replace it and you're done.

If the diaphragm is good, put it back together and unplug the monovalve and drive it. See if you then have heat.