How to Troubleshoot a Thermostat Y Wire on an Air Conditioner
The AC portion of your heating, ventilating and air-conditioning system is controlled by the thermostat and the Y wire. If your system is not turning on or off at the desired temperature, the problem could be in the thermostat or the Y wire. Once you know how to troubleshoot the Y wire on an air-conditioning system, you will be able to determine where the problem is located. Does this Spark an idea?
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Instructions
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Check the transformer to see whether it has power. The quick way to perform this check is to set the fan mode to "on" at the thermostat. If the fan comes on, the transformer is getting power. If the fan does not come on, the problem is in the power supply and not the Y wire.
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Disconnect the power to the HVAC system by turning the circuit breaker off to the system at the home electrical box.
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Use a flathead screwdriver to pop the face cover off of the thermostat. There is a slot between the face of the thermostat and the thermostat body for the blade of the screwdriver.
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Use a Phillips-head screwdriver to unscrew the mounting screws that secure the thermostat to the wall.
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Check the connection of the Y wire inside the thermostat. It may be broken or some of the wire insulation may be jammed between the terminal screw and the bare portion of wire.
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Wrap small pieces of masking tape on each wire and label the wires. Remove the wires from the thermostat by unscrewing the terminals with a screwdriver. Bend the wires around a pencil to prevent them from slipping into the wall cavity.
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Twist the R wire and Y wire together. Make sure that the other wires are not touching one another.
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Turn the circuit breaker on to the HVAC system. If the air conditioning unit comes on, the problem is within the thermostat and it needs to be replaced. If it does not come on, the fault is either a short in the Y wire or a bad relay.
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