Thermostat shows E04 and reports an external room sensor fault when using a remote Bosch NTC room sensor.

Safety: Switch off power to the HVAC system at the breaker before accessing thermostat or sensor wiring.

• Verify configuration: In the installer/advanced menu, confirm that an external room sensor is enabled only if one is actually installed, and that the correct sensor type is selected.
• Check connections at thermostat: Remove the thermostat from the wall plate and verify the remote sensor wires are firmly secured in the designated sensor terminals (often labeled S1/S2 or similar).
• Inspect sensor wiring run: Follow the cable from thermostat to the remote sensor. Look for staples, pinched sections, or splices that may have caused a short or break.
• Check sensor at field end: Remove the remote sensor cover and ensure the wires are tight on the sensor terminals. If available, measure resistance with a multimeter (power off). A Bosch NTC sensor should show a finite resistance (not 0 Ω or infinite). If it reads open or short, replace the sensor.
• Replace sensor if needed: If wiring is intact but E04 remains, install a new Bosch remote room sensor of the correct type.