Viessmann · Selected Vitodens and Vitotronic ranges
18 / D1Outdoor temperature sensor open-circuit; on Vitodens 100-W may indicate DIP switch S2 is on
This code applies to Vitodens 100-W, Vitodens 200-W, Vitodens 222-F, Vitotronic 100, Vitotronic 200, Vitotronic 200-H, Vitotronic 300, Vitotronic 300-K, Vitocontrol-S. Fault codes narrow down the likely problem, but they do not replace a model-specific diagnosis.
What you can safely check
- No internal homeowner repair is safe for this fault.
- A single reset may be tried only where the model's user manual allows it.
- Arrange an engineer if the code remains or returns.
How an engineer may fix it
Test the relevant sensor or thermistor, its position, resistance, plug and wiring; repair the connection or replace the failed sensor and verify operation.
Viessmann warns that the same code can have different meanings on different models. The exact manual must take precedence.
Manufacturer source: Viessmann UK — Fault Code Checker. Checked 15 August 2026. The appliance manual and an engineer’s on-site diagnosis take precedence. Never remove the boiler case yourself.