21
Vokera
Communication Fault with External Controller
Display shows 21, furnace may run at incorrect output or not respond to modulation
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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
Damaged OpenTherm/eBus cable, Incorrect polarity, Incompatible controller, PCB communication fault
How to Fix / Troubleshooting
Safety: Isolate power before working on control wiring.
- Check controller type: Ensure the connected controller is approved for use with Vokera furnaces and matches the communication protocol (e.g., OpenTherm).
- Inspect cable: Look for damage or loose connections on the low-voltage communication cable between the controller and furnace.
- Polarity: Some buses require correct polarity. Verify wiring matches the installation manual.
- Test with simple thermostat: Temporarily connect a basic on/off thermostat to see if the furnace operates normally. If so, the advanced controller or its wiring is at fault.
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Repair Difficulty
Medium
3/5
Required Part
External Controller Interface
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