Furnace shows A21; burner does not start or shuts down quickly
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.
Possible Causes
How to Fix / Troubleshooting
Safety: Switch off power and ensure the appliance is cool. Do not operate the furnace with a known flue blockage.
- Check air intake and flue: From outside, inspect the concentric flue terminal for leaves, nests, ice, or debris. Clear any obstruction. Ensure terminal clearances meet Bosch installation requirements.
- Inspect combustion fan: Remove the front cover and visually inspect the fan wheel for dust buildup or damage. Spin the wheel gently by hand (power off) to confirm it turns freely.
- Air pressure switch tubing: Check silicone tubes from the fan or flue to the air pressure switch for cracks, kinks, or water. Replace damaged tubing and drain any condensate.
- Restart and listen: Restore power and call for heat. Confirm the fan starts and ramps up. If the fan hums but does not spin, the fan motor may be failing and should be replaced.
If A21 persists after clearing the flue and checking tubing, a technician should test the fan, pressure switch, and control board.
Repair Difficulty
Required Part
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