A91
Buderus
System Sensor / Additional NTC Fault
Error A91 appears; heater may operate erratically or not at all.
Advertisements
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.
Advertisements
Possible Causes
Failed system temperature sensor on primary loop, Sensor not properly seated in well, Wiring damage due to heat or abrasion, Incorrect sensor mapping in control settings
How to Fix / Troubleshooting
Safety: Turn off power and allow system to cool before touching piping or sensors.
- Identify system sensor: Consult the Buderus wiring diagram to locate the system or primary loop sensor (often on the main flow or return line).
- Check physical installation: Ensure the sensor is fully inserted into its well or clamped securely to the pipe with thermal paste or insulation as specified.
- Inspect wiring: Look for brittle insulation, melted spots, or pinched cables. Repair or replace as needed.
- Measure resistance: Disconnect the sensor and measure resistance. Compare to Buderus NTC tables. Replace if out of tolerance.
- Verify control configuration: In the installer menu, confirm that the sensor is assigned to the correct function and that unused sensor inputs are disabled.
- Restore and test: Reassemble, restore power, and confirm that the A91 code clears and system temperatures are read correctly.
Advertisements
Repair Difficulty
Medium
3/5
Required Part
System Temperature Sensor (NTC)
Check Price on Amazon
We may earn a commission from links on this page.
Advertisements