Unit displays A11 and will not operate; may occur after power interruption or wiring work.

Safety: Turn off power to the heater before working on low-voltage wiring.

• Identify if outdoor sensor is used: Check the installation manual or wiring diagram. Some Buderus water heaters or combi units use an outdoor sensor for weather-compensated control.
• Inspect outdoor sensor: Locate the sensor on the exterior wall. Check for physical damage, water ingress, or paint covering the sensor.
• Check wiring run: Follow the low-voltage cable from the sensor back to the heater. Look for staples through the cable, cuts, or UV damage.
• Measure resistance: Disconnect the sensor at the heater and measure resistance. Compare to Buderus outdoor sensor specs at ambient temperature. Replace if out of range.
• Verify polarity and terminals: Ensure the sensor wires are connected to the correct terminals on the control board as per the wiring diagram.
• Disable feature (if allowed): If no outdoor sensor is intended, confirm in the control parameters that the outdoor sensor function is disabled according to the service manual.
• Restore power and test: Turn power back on and verify that the A11 code clears and normal operation resumes.