cardy, what jimmy is saying is to check from common to wire from thermostat to see if it reads 24 VAC which would indicate its closed. If it's open there will be no reading (0 VAC). The thermostat wires go up to the air handler (inside unit usually in attic) ergo check at air handler. Once you have determined that the thermostat is operating correctly you would then check the contactor (electic relay switch) in the outside unit to see if it is in the closed position. If it is in the closed position and there is no 24 VAC present across the coil, it's hung up. Contactor should be normally open. That means when no 24 VAC to coil contact stays open, when voltage applied it closes. As a practical matter you can avoid going to the attic by making your checks at the outside unit. Set thermostat temperature about 10 degrees above room temperature (thermostat will not call for cooling since room temperature is 10 degrees below what you have set it for) and check contactor (relay) at outside unit for voltage on coil. If none present, thermostat is OK. If contactor closed with no 24 VAC its hung up. You could try pushing the contactor open using light pressure and using a non-coductive pry (wood popcicle stick, plastic straw) which may get it to operate again but its bound to get hung up again so it should be replaced. PL
Last edited by PatrickL1633 : 11-03-2007 at 06:52 PM.
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