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I am getting no power to thermostat or rooftop unit. Thermostat is a 800435-3, tested same thermostat and another room and it works perfect. Furnace Board Status LED is off. Tried resetting breaker and still have no power. I have searched around and have seen that this is sometimes caused by a limit switch?
 

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Check the low voltage fuse.

If it's not that, will have to be checked with a meter - could be anything.

When the high limit is open with power to the unit, blower runs continuously.

Only old units completely shut low voltage power when limit opens.
 

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That unit doesn't have the high limit in the circuit between the transformer and the board's power. Either the fuse on the board is blown, the transformers circuit breaker is tripped, the transformer is worn out, or you don't have line voltage to the unit.
 
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