Are you absolutely sure it's the control head? The most common failure point of these systems has been leaky vacuum hose connections. They start to get brittle and crack where they are attached to the various actuators. Your system is a few years older than several that have had failures (mine included).
The one that usually fails is the connection to the actuator that controls/modulates the hot water. On the under hood side of the dash, look for a green vacuum line attached to a valve actuator with 2 hot water hoses attached. Look very closely where the vacuum line attaches to the actuator - you may find a split hose.
Take a look at the picture on
this page...
It seems to happen more on the diesels, but I believe you have the same SCS system.