Concord Low-Voltage Power Drop or Short

Thermostat goes blank when furnace starts, then comes back on after a while.

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Short in thermostat wiring, Blown low-voltage fuse on control board, Transformer overload, Outdoor unit contactor coil short (for combined systems)

Safety: Turn off power before working on low-voltage circuits.

Steps:

Inspect thermostat cable: Look for staples driven through the cable or damaged insulation along its run.
Check 24V fuse: Replace the fuse on the Concord control board if blown. If it blows again, disconnect thermostat wires and reconnect one by one to isolate the shorted circuit.
Check outdoor unit: If the furnace shares a transformer with an AC condenser, disconnect the Y wire to the outdoor unit and test. A shorted contactor coil can pull down the transformer.

Medium 3/5

24V Transformer

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