C1
Bosch
Air Pressure Switch Not Closing
Furnace shows C1; burner does not start, fan may run briefly
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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
Blocked flue or intake, Defective air pressure switch, Disconnected or leaking pressure tubing, Combustion fan not running or running too slowly
How to Fix / Troubleshooting
Safety: Turn off power and gas. Never bypass the air pressure switch for testing while operating the furnace.
- Inspect flue and intake: Check both intake and exhaust pipes for obstructions, sagging, or water accumulation. Clear any blockage and correct pipe slope.
- Check tubing: Remove the silicone tubes from the air pressure switch and fan. Blow gently through them to ensure they are clear. Replace if brittle or cracked.
- Test switch operation: With power off, disconnect the wires from the pressure switch and measure continuity while gently applying suction with a hand pump. The contacts should close. If not, replace the switch.
- Verify fan operation: Restore power and call for heat. Confirm the fan starts before C1 appears. If the fan does not run, the fan or control board may be faulty.
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Repair Difficulty
Hard
4/5
Required Part
Air Pressure Switch
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