Bosch EasyControl displays error C17 and indicates that the external temperature sensor (for weather compensation) is not detected or faulty.

Safety first: Isolate power to the boiler and any external sensor circuits before working on wiring.

• 1. Locate sensor: Find the Bosch outdoor temperature sensor mounted on an external wall, usually north-facing and away from direct sun.
• 2. Inspect physically: Check for physical damage, water ingress, or loose cable glands. Ensure the cable is firmly connected at both the sensor and boiler/EasyControl interface.
• 3. Test resistance: With power off, disconnect the sensor and measure its resistance with a multimeter. Compare to the Bosch temperature/resistance chart. Replace if readings are out of range.
• 4. Check wiring continuity: Test each conductor from sensor to boiler for continuity and absence of shorts.

Professional help: If wiring and sensor are good but C17 persists, the boiler’s sensor input or EasyControl interface may be faulty and should be checked by a technician.