Continuous Fast Flash
Trane Incorrect 24V Phase / Low Voltage Polarity

Status LED flashes rapidly without pause, furnace does not respond to thermostat

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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

R and C low-voltage leads reversed, Shared transformer with incorrect phasing, Miswired thermostat connections, Short between R and C

How to Fix / Troubleshooting

Safety: Turn off furnace power before working on low-voltage wiring. Although 24V is low, short circuits can damage the control board.

Steps to check:

  • Inspect thermostat wiring: Remove the blower door and locate the low-voltage terminal strip (R, C, W, Y, G). Ensure wires are firmly attached and not shorting between terminals.
  • Check for shorts: Look for bare thermostat wires touching the furnace cabinet or each other. Repair with electrical tape or replace damaged cable.
  • Verify R and C: Confirm that the thermostat and any outdoor unit are wired consistently to R and C. Mixed phasing between multiple transformers can cause fast flash errors.
  • Reset control: After corrections, restore power and observe LED status.

When to call a professional: If the fast flash persists, a technician should check transformer phasing, measure 24V circuits, and possibly replace the control board.

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Repair Difficulty

Medium 3/5

Required Part

Low Voltage Thermostat Wiring
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