Display shows E3 and unit indicates sensor bus error (for models with multiple sensors)

Safety first: Disconnect power before accessing internal wiring.

• Inspect bus wiring: Trace the sensor bus cable between the control board and connected sensors (flow, temperature, salt level). Look for damage or loose connectors.
• Disconnect sensors one by one: With power off, disconnect one sensor at a time and then power up to see if E3 clears. This can help identify a faulty sensor pulling down the bus.
• Check addressing (if applicable): Some sensor modules have DIP switches or jumpers for addressing. Verify they are set according to the installation manual.
• Replace faulty sensor: Once the problematic sensor is identified, replace the corresponding Viessmann sensor module.
• Board check: If all sensors and wiring are good, the control board bus interface may be defective and require replacement.