Verified boiler fault code
Viessmann 18 Fault Code (Vitodens/Vitotronic fault-code checker row for outdoor sensor)
Outdoor temperature sensor cable broken/open circuit.
Safety disclaimer
Fault code summary
- Manufacturer
- Viessmann
- Fault code
- 18
- Fault type
- Temperature sensor, Electrical
- Severity
- Not specified
- Reset
- Not specified
- Applicable models
- Vitodens 100, Vitodens 200-W, Vitodens 222-F, Vitotronic 300-K, Vitocontrol-S, Vitotronic 100, Vitotronic 200, Vitotronic 300, Vitodens 050/200-H
Quick answer
Outdoor temperature sensor cable broken/open circuit.
What does this fault code mean?
Outdoor temperature sensor cable broken/open circuit.
Common causes
- Outdoor temperature sensor cable broken or open circuit.
Symptoms
- The boiler may operate as if the outdoor temperature were 0°C/32°F or in constant mode, depending on the listed control.
What can the homeowner safely check?
This information was not provided in the verified manufacturer source.
Boiler reset information
Reset information was not provided in the verified manufacturer source. Do not repeatedly reset an unresolved fault.
When should I call an engineer?
Contact a suitably qualified heating engineer when the fault persists, the boiler locks out, or the manufacturer instructions do not resolve it. For gas appliance work in the UK, use a Gas Safe registered engineer.
Engineer diagnosis
- Assess the outdoor sensor cable and connections.
Applicable boiler models
Related fault codes
Manufacturer documentation
Viessmann UK Fault CodesFault Code 18 and D1 | Vitodens 100 / 200-W / 222-F / Vitotronic 300-K With 100 / Vitocontrol-S | Outdoor temperature sensor cable broken
Last verified
2026-09-12
Frequently asked questions
What does Viessmann 18 mean?
Outdoor temperature sensor cable broken/open circuit.