Fraser-Johnston Loss of 24V Control Power

Thermostat display is blank and system does not respond

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Safety Warning

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.

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Possible Causes

Blown low-voltage fuse on Fraser‑Johnston control board, Shorted thermostat cable, Failed 24V transformer, Loose R or C connections

How to Fix / Troubleshooting

Safety: Turn off power to the air handler/furnace before accessing the control compartment.

  • Check board fuse: Inspect and replace the low-voltage fuse on the control board if blown.
  • Inspect thermostat cable: Look for staples through the cable or damaged insulation causing shorts between conductors.
  • Test transformer: Measure voltage at the transformer secondary. If no 24 VAC is present, replace the transformer with the correct rating.
  • Verify connections: Ensure R and C wires are securely connected at both the thermostat and control board.
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Repair Difficulty

Medium 3/5

Required Part

24V Transformer
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