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Brian HVAC Guy
Brian HVAC Guy, HVAC Technician
Category: HVAC
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Experience:  I have been an HVAC/R mechanic/technician for 30+ years.
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THM5421C no output to 1-2-3 Com terminals
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Customer Question

Customer: THM5421C no output to 1-2-3 Com terminals
JA: Thanks. Can you give me any more details about your issue?
Customer: i was moving my honeywell TS from one wall to another and upon reconnecting it will not power back on thinking I may have shorted out the Honeywell EIM
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Submitted: 1 year ago.
Category: HVAC
Expert:  Brian HVAC Guy replied 1 year ago.

Hi I'm Brian and I'm here to help!

Chances are the EIM is ok. It's more likely that the fuse or low voltage transformer on the heating or cooling unit the EIM is powered from is blown. You should check for 24 volt ac power on the transformer in the unit that sends power to the EIM on terminals R and C below the U terminals on the EIM. If no power there then you should look at possibly a control fuse in the air handler or furnace that's being controlled or if no fuse, look at the transformer itself.

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