Bosch EasyControl display shows error C1 and heating does not start or stops shortly after demand.

Safety first: Turn off power to the boiler and the EasyControl at the isolation switch or breaker before touching any wiring. Verify with a non-contact voltage tester where appropriate.

Step-by-step checks:

• 1. Verify wiring type: Confirm that the boiler supports OpenTherm and that the EasyControl is wired to the dedicated OpenTherm terminals (often marked OT, OT+, OT−, or +/-). Do not use 230 V thermostat terminals.
• 2. Check polarity: On many Bosch boilers, OpenTherm is polarity-insensitive, but some installations can be affected by incorrect connection. Swap the two OpenTherm wires on the boiler side and retest after restoring power.
• 3. Inspect cable and joints: Follow the low-voltage cable from the EasyControl wall plate to the boiler. Look for damaged insulation, crushed cable, or loose screw terminals. Re-strip and re-terminate any suspect conductors.
• 4. Confirm boiler configuration: In the boiler’s installer menu, ensure the control type is set to OpenTherm or modulating room controller (refer to the boiler manual). If it is set to on/off thermostat, change to OpenTherm and save.
• 5. Power cycle: Turn power back on, wait 2–3 minutes, and check if C1 clears. Try a call for heat from the EasyControl app or thermostat.
• 6. Isolate the fault: If possible, temporarily connect a short known-good 2-core cable directly between the EasyControl and boiler OpenTherm terminals. If C1 disappears, the original cable run is faulty. If C1 persists, the boiler’s OpenTherm interface or the EasyControl main board may be defective.

When to call a professional: If boiler configuration appears correct and a known-good cable does not resolve the issue, a heating engineer or Bosch service technician should test the boiler’s OpenTherm module and the EasyControl electronics.