Trane Insufficient Control Voltage / Backlight Power Issue

Thermostat has power but screen is very dim or backlight flickers

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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

Weak 24 VAC transformer, Excessive load on control circuit (too many accessories), Loose C wire connection, Failing thermostat backlight driver

How to Fix / Troubleshooting

Safety: Turn off power to the HVAC system before adjusting wiring.

  • Step 1 – Measure control voltage: With a multimeter and power ON, measure between R and C at the thermostat. It should be around 24 VAC. If below 20 VAC, suspect transformer or wiring issues.
  • Step 2 – Tighten C connections: Turn power OFF and tighten the C terminal screws at both the thermostat and indoor control board.
  • Step 3 – Reduce accessory load: If multiple accessories (humidifier, dehumidifier, ERV) are powered from the same transformer, consider adding a separate 24 VAC transformer for accessories.
  • Step 4 – Replace transformer if weak: If the transformer output is consistently low under load, replace it with a properly rated Trane transformer.
  • Step 5 – If voltage is normal but screen remains dim: The thermostat’s backlight circuitry is likely failing; replace the thermostat.
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Repair Difficulty

Medium 3/5

Required Part

Trane Indoor Unit Transformer or Trane Smart Thermostat
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