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Even though my thermostat was set to "Off" or "Heat", my air conditioner is coming on randomly and blowing cold air out of the vents. I figured that the thermostat was bad so I disconnected all of the wires to replace it, but even without a thermostat attached, it's still blowing cold air.

I have an upstairs and a downstairs unit, but only the downstairs unit has this problem.

What else could cause the AC to be turning on?

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up vote 5 down vote accepted

Probably a problem with the control board on the AC unit. If the AC unit comes on without a signal from the thermostat, there must be a short or something that is giving the AC unit the "turn on" signal.

Contact the company that installed the unit (if it's still under warranty), or your favorite HVAC company to come take a look.

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Thanks for the info. I had never heard of that kind of failure before. – David Nov 24 '10 at 16:20
This exact problem happened to me once in a rental. The thermostat was fine, but the relay in the control board at the furnace was bad. – Evil Elf Jun 7 '11 at 12:38

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