Boiler displays E12, no heating or hot water. Often associated with external control or thermostat issues.

Safety first: Isolate power before working on control wiring. Low-voltage circuits can still be damaged by short circuits.

Step-by-step checks:

• 1. Check thermostat settings: Ensure the room thermostat or Viadrus controller is calling for heat (temperature set above room temperature, program not in OFF mode).
• 2. Inspect wiring: Remove the boiler front panel and locate the room thermostat terminals. Check that wires are firmly connected and not broken.
• 3. Temporary link test (for volt-free contacts only): With power off, remove the thermostat wires and fit a small link wire between the two thermostat terminals. Restore power. If the boiler now runs without E12, the thermostat or its cable is faulty.
• 4. Replace or repair thermostat: If the link test proves the boiler is fine, replace the room thermostat or repair the cable run.

When to stop: If E12 persists even with a link fitted, the PCB’s control input may be defective and requires professional diagnosis.