Follow the heater hose from the thermostat housing to the firewall and locate the return hose. Is the return hose cold? Is the supply hose warm, hot, or cold? If warm or hot follow it to a valve and feel the opposite side of temp. Your system may not be so simple but it should have a hot hose to the heater. If it is hot both in and out the damper door on a plenum is not opening.
Make sure as well that there is no air in the cooling system.
Sunday, February 4th, 2007 AT 11:36 AM