A3
Viessmann
Combustion Optimization Warning
A3 warning, fan speed or combustion values out of optimal range but unit still runs
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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
Partially blocked burner, Slight flue restriction, Fan speed drift, Gas valve modulation slightly out of calibration
How to Fix / Troubleshooting
Safety: Turn off power and gas before opening the combustion chamber. Combustion adjustments must be done with proper instruments.
- Clean burner: Remove and clean the burner surface with a soft brush and vacuum. Do not use sharp tools that can damage the burner.
- Inspect flue: Check for partial obstructions or sagging sections that may affect draft.
- Fan check: Ensure the fan impeller is clean and spins freely. Replace if noisy or imbalanced.
- Professional combustion setup: Have a qualified technician perform a combustion analysis and adjust gas valve settings as needed to bring values back into specification.
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Repair Difficulty
Professional Required
5/5
Required Part
Burner Assembly
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