A4
Miller
Flame Sense Error (Advanced Board)
Miller furnace digital display shows A4; unit may short-cycle or not start.
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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
Contaminated flame sensor, Poor burner ground, Low microamp flame signal, Faulty control board flame circuit
How to Fix / Troubleshooting
Safety: Turn off power and gas before removing the burner assembly or flame sensor.
- Clean flame sensor: Remove the sensor and polish the rod with fine abrasive pad. Avoid touching the rod with bare fingers afterward.
- Check ground path: Ensure the burner rack is tightly fastened to the furnace chassis and that ground wires from the control board are secure.
- Measure flame signal: A technician can measure flame rectification current (microamps) in series with the sensor lead. If the signal is below manufacturer spec, clean burners and sensor or adjust gas/air mixture as needed.
- Replace sensor or board: If cleaning and grounding do not restore proper operation, replace the flame sensor. If still faulty, replace the control board.
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Repair Difficulty
Hard
4/5
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