Trane
Voltage Sag on Control Circuit
Thermostat backlight or display flickers when HVAC equipment starts
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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
Undersized or failing transformer, Loose neutral or ground on HVAC circuit, High inrush current from blower or compressor, Shared transformer powering many accessories
How to Fix / Troubleshooting
Safety warning: Electrical measurements should be performed by someone comfortable working around live circuits.
- Step 1 – Observe pattern: Note if flicker occurs only when blower or compressor starts.
- Step 2 – Measure control voltage: With a multimeter, monitor R–C voltage while equipment starts. Significant dips below 18 VAC indicate a problem.
- Step 3 – Tighten connections: With power OFF, tighten all low-voltage connections at the indoor control board and thermostat.
- Step 4 – Upgrade transformer: If the transformer is undersized for the load, replace it with a properly rated Trane transformer.
- Step 5 – Separate accessory power: Consider powering accessories from a separate transformer to reduce load on the thermostat circuit.
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Repair Difficulty
Hard
4/5
Required Part
Trane Indoor Unit Transformer
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