Boiler shows E20, burner does not start, room thermostat appears to call for heat.

Safety: Turn off power before modifying control wiring.

• Check thermostat demand: Ensure the room thermostat or Klimosz controller is set above room temperature and in heating mode.
• Inspect wiring: Verify continuity of the thermostat cable between the boiler terminals and the room unit. Tighten all terminal screws.
• Bypass test: Temporarily bridge the thermostat terminals at the boiler (if using simple on/off control). If the boiler starts, the thermostat or its wiring is faulty.
• Bus systems: For eBUS or proprietary Klimosz controllers, check that the correct controller type is selected in the boiler menu and that polarity (if required) is correct.
• Replace controller: If wiring is sound and E20 persists, replace or reprogram the room controller according to Klimosz instructions.