Boiler shows E19, burner does not start, and there may be no response to external demand signals.

Safety first: Isolate power before altering control wiring. Use only compatible Viadrus-approved controllers on digital communication terminals.

Step-by-step checks:

• 1. Check controller power: Ensure the OpenTherm or digital controller is powered and displaying correctly.
• 2. Inspect wiring: Verify that the two OpenTherm wires are correctly connected to the designated terminals on both boiler and controller, with no shorts or breaks.
• 3. Test with simple thermostat: If possible, temporarily switch the boiler to basic on/off thermostat mode (see manual) and connect a simple room thermostat. If the boiler works, the OpenTherm controller or wiring is at fault.
• 4. Reset both devices: Power down both boiler and controller, then power up the boiler first, followed by the controller.

When to stop: If E19 persists with known-good wiring and controller, the boiler’s communication interface may be defective and needs professional service.