Sounds like the monovalve isn't getting any power. The default setting is full heat.
When the system is modulating heat, I believe there's a pulsed ground signal to the monovalve. The other wire is constant 12V in run or start.
Could be as complex as a bad temperature control module or as simple as a bad connection between the control module and the monovalve.
Don't discount the push button unit as the temp controller signal goes through the push button unit to be ignored (for full heat) if defrost mode is selected. But I believe there's a mechanical link between the vent selector and the monovalve mode selector inside the push button unit.
Actually, it could be as simple as a fuse but on a 126 (that's all I'm familiar with) the same fuse powers the push button unit and the monovalve.