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I have a Lennox system (G27M series). The transformer has a 3amp breaker. It trips time when the R wire and W wire are connected (heat is on).

To find out where the circuit is short, I attempted to measure the resistance between R wire and W wire, and found the reading is zero ohm (thermostat was taken off and furnace was unplugged). Is that normal? Is it a sign that the control board is broken?

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  • If you remove the W wire from the control board terminal and recheck the resistance of the thermostat R and W wires, is it still 0 ohms? – Dotes Nov 2 '17 at 11:59
  • Where did you measure the wires? Coming out of the wall at the thermostat, inside the air handler? Where? – JPhi1618 Nov 2 '17 at 14:43
  • @JPhi1618 I measured the resistance at the control board inside the furnace. – zhanxw Nov 2 '17 at 15:16
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    @zhanxw maybe I'm misunderstanding what you're saying, but 0 ohms across two different thermostat wires that aren't connected to anything means there is a short in the wires. The resistance between the control board terminals sounds about normal. – Dotes Nov 3 '17 at 11:06
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    @Jeff the problem is that the secondary limit switch (installed inside the blower) is connected to the ground and caused the shortage. Thanks. – zhanxw Nov 6 '17 at 21:10

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