E15
CTC
Low-Voltage Transformer Failure
Furnace indicates low 24 V transformer output or control voltage fault.
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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
Failed 24 V transformer, Short in thermostat wiring, Blown low-voltage fuse, Shorted gas valve or contactor coil (if AC shared)
How to Fix / Troubleshooting
Safety first: Turn off power before testing or replacing the transformer.
Steps:
- Locate the CTC 24 V control transformer, usually mounted near the control board.
- Inspect the low-voltage fuse on the control board; replace if blown, but investigate for shorts before restoring power.
- With power OFF, visually inspect thermostat and low-voltage wiring for damage, staples through cable, or bare wires touching metal.
- With power ON (only if trained), measure transformer secondary voltage between R and C; it should be around 24 VAC. If zero or very low, and primary voltage is present, the transformer is likely failed.
- Replace the transformer with a CTC-approved unit of the same VA rating and voltage.
Warning: A recurring blown fuse or failed transformer indicates an unresolved short; professional troubleshooting is recommended.
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Repair Difficulty
Hard
4/5
Required Part
CTC 24V Control Transformer
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