Been chasing a problem with the system for about a month. It intermittantly goes to full heat, rolling the temp knob to full cold restores the system for 15-30 minutes then again full heat. The monovalve was replaced about a year ago, my first suspicion was that it failed again. Looked at the diaphragm, saw no tears so put it back in. Suspecting a leaking vacuum actuator I checked all them, but they all held. Next I checked both the temp sensors, they met spec. Scratching my head I again went to the monovalve. This time I totally disassembled the diaphragm assembly. The rubber was still intact, but the insides were flooded. My suspicion is that the tiny rubber sealing ring under the "C" clip failed. Installed a monovalve repair kit and life is good again. Moral of the strory, if the system is going to full hot, chances are it is the monovalve. A close inspection of the entire valve, not just the rubber diaphragm, should be the first troubleshooting step.