C6
Bosch
Fan Speed Calibration / Combustion Control Error
Bosch tankless water heater shows C6; fan speed or combustion control issue
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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
Incorrect fan calibration, Fan speed sensor malfunction, Control board firmware issue, Venting outside design parameters
How to Fix / Troubleshooting
Safety first: Turn OFF power and gas. Combustion adjustments affect safety and emissions.
- Check venting: Confirm that vent length, diameter, and termination meet Bosch specifications. Correct any obvious installation issues.
- Inspect fan and wiring: Ensure the combustion fan spins freely and that its wiring harness is secure.
- Reset / recalibrate (if supported): Some models allow a fan calibration or reset procedure via the control panel. Follow the service manual instructions exactly.
- Professional service: If C6 persists, a technician should verify combustion with a flue gas analyzer and replace the fan or control board if necessary.
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Repair Difficulty
Professional Required
5/5
Required Part
Main Control Board (PCB)
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