Bosch EasyControl shows error C4 and cannot detect or control the heating circuit correctly.

Safety first: Isolate power to the boiler and EasyControl before inspecting wiring or sensors.

• 1. Review system configuration: In the EasyControl installer menu, verify that the correct system type is selected (e.g., combi boiler, system boiler with cylinder, underfloor heating). Correct any mismatches and save.
• 2. Check boiler sensors: On compatible Bosch boilers, ensure the flow and return temperature sensors are connected and not damaged. A missing or open-circuit sensor can cause the EasyControl to flag C4.
• 3. Inspect EasyControl location: Make sure the thermostat is not installed in a location that causes sensor misreading (e.g., above a radiator, in direct sunlight, or near a draught). Relocate if necessary using the Bosch EasyControl wall plate and low-voltage wiring.
• 4. Multi-zone setup: If using Bosch EasyControl Smart Radiator Thermostats, verify that zones are correctly assigned in the app and that each device is paired and online. Incorrect zone mapping can trigger configuration errors.

Professional help: If boiler sensors test correctly and configuration is correct but C4 persists, a technician should test the EasyControl internal sensor and the boiler’s sensor circuits.