Customer's Heatilator model GCDC60 worked fine for 11 years. A couple of months back it started not turning on by the wall thermostat. Then it wouldn't turn off unless customer turned the Robershaw 7000 gas valve control to "off". I have disengaged wall thermostat, and it stil won't turn off unless I disconnect one of the two thermostat wires from the control valve, even though the wall thermostat is off and it's two connecting wires aren't touching! Pilot lite is on, thermopile must be working, because the main burner continues working after it starts up, then the fan blower motor kicks in when it's supposed to start distributing hot air into the room.
Have you tried doing a continuity test on the wire with it unhooked from the gas valve. Maybe it shorted through the coating on a staple in the wall, or a nail or something. You may need to run a new wire to the stat if that is the case. Just a suggestion.