How to Fix / Troubleshooting

Safety: Turn off power. Hot water lines and tank surfaces can be very hot; use gloves.

Steps:

Locate DHW sensor: On Buderus systems with indirect tanks, the DHW sensor is usually inserted into a dry well on the tank. On combi units, it may be clipped to the DHW outlet or integrated into the hydraulic block.
Check connection: Verify the sensor plug is firmly connected to the Buderus control board and that the cable is intact.
Inspect for moisture: Look for signs of water ingress around the sensor well or cable entry. Dry the area and correct any leaks before replacing the sensor.
Test sensor: With power off, measure resistance and compare to Buderus charts. Replace if out of range.

After replacement, clear the error by cycling power and confirm stable DHW operation.

Furnace shows A9, may limit output or shut down on temperature error

Possible Causes