My HVAC stopped working, no matter what state the thermostat is in there is no fan, no compressor, no heat, etc. Using a multimeter, I found that the output voltage on the transformer was 0V. Unplugging the transformer, I checked the voltage across the two blade connectors. The power side shows 117V. So I went ahead and ordered a replacement transformer.

After installing the new transformer, the HVAC still was not working. I checked the output on the new transformer and it is still 0V. Is the new transformer faulty? What else should I be testing? When I plugged the unit in, there was no smoke or smells that may indicate the transformer being fried.

Using the multimeter, and testing the transformer independant of the circuit, the high side is showing 20 ohms resistance, the low side shows 1.6 ohms. Checking the wires that shouldn't be connected internally shows no continuity.

UPDATE: The furnace control board is The furnace control board is CESO110074-01 https://arnoldservice.com/product/ceso110074-01-bryant-carrier-furnace-control-circuit-board/

The furnace is a Carrier Product: 58gfa085---101MC Model: 58gfa085-mc

  • Unplug the thermostat and check to see if there's any voltage on the transformer. Commented Dec 25, 2016 at 2:30
  • Yeah, I unplugged the thermostat before checking the voltage across the transformer's output leads.
    – DavGarcia
    Commented Dec 25, 2016 at 2:47
  • Take an ohmmeter reading between the two terminals on the HVAC system that go to the transformer while the transformer is unplugged from the system, then. If you can get us a wiring diagram or make/model of this system, that'd help a ton, too. Commented Dec 25, 2016 at 3:04
  • Checking just across the connectors on the control board, with the transformer disconnected - there is no continuity between PR-1 and PR-2, the two quick connects for the transformer's input. Out of curiosity, I checked SEC-1 and SEC-2 which is where the low voltage plugs back into the board and that shows 11.6 ohms.
    – DavGarcia
    Commented Dec 25, 2016 at 3:15
  • The furnace control board is CESO110074-01 arnoldservice.com/product/…
    – DavGarcia
    Commented Dec 25, 2016 at 3:15


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