How to Fix / Troubleshooting

Safety: Turn off power before working on venting or fan components.

Steps:

Check intake and exhaust: Inspect the outdoor terminations of the Buderus furnace vent system. Remove snow, leaves, nests, or other obstructions from both intake and exhaust pipes.
Inspect fan wheel: Access the combustion fan and check for heavy dust, rust, or broken blades. Clean the wheel thoroughly and ensure it is firmly attached to the motor shaft.
Verify condensate drainage: On condensing Buderus furnaces, a blocked condensate trap can flood the combustion chamber and affect fan speed. Clean the trap and hoses and ensure proper slope.
Reset and test: After cleaning and clearing obstructions, restore power and test operation. If C7 returns, the fan motor or speed feedback circuit may be defective.

Fan speed calibration and replacement should be handled by a qualified technician.

Display shows C7, furnace shuts down shortly after fan starts

Possible Causes