Bosch EasyControl shows error C2 and cannot control the boiler; heating remains off or runs only in basic on/off mode.

Safety first: Isolate mains power to the boiler and control system before opening any covers.

• 1. Confirm compatibility: Check the boiler model against Bosch EasyControl compatibility lists. Some older or non-Bosch boilers may not support OpenTherm or may require an additional interface module.
• 2. Verify terminal connections: Ensure the EasyControl is connected to the boiler’s low-voltage bus/OpenTherm terminals, not to 230 V thermostat or relay contacts. Re-seat the wires and tighten terminal screws.
• 3. Check boiler settings: In the boiler’s installer menu, enable the external controller / OpenTherm / EMS bus as required. Some boilers default to internal control and must be switched to external control.
• 4. Inspect for damage: Look for signs of burning, corrosion, or water ingress on the boiler’s control board around the bus terminals. Do not attempt board-level repair; note any damage for a technician.
• 5. Test with another controller (if available): If you have access to a known-working OpenTherm controller, connect it temporarily to see if the boiler responds. This helps determine whether the issue is with the boiler or the EasyControl.

Professional help: If the boiler bus is damaged or the EasyControl still reports C2 after correct wiring and configuration, contact a qualified heating engineer or Bosch service to test and possibly replace the boiler interface or EasyControl unit.