Thermostat displays E09 and cannot detect a required C‑wire; device may run only on batteries or not power up fully.

Safety: Turn off power to the HVAC equipment at the breaker before working on thermostat wiring.

• Check thermostat backplate: Confirm that a wire is connected to the C terminal. If not, look for an unused conductor in the cable that can be repurposed as C.
• Inspect at equipment: At the Bosch boiler/furnace control board, locate the C terminal and ensure the same color wire used at the thermostat C is connected there.
• Verify continuity: If you have a multimeter, with power off, check continuity of the C conductor between thermostat and control board. Replace or re‑use another conductor if the original is broken.
• Use a Bosch‑approved power module: If no spare conductor is available, consult Bosch documentation for an approved external 24 V power supply or C‑wire adapter compatible with your thermostat model.
• Secure all connections: Tighten terminal screws and ensure no bare copper is exposed that could short to other terminals.