Viessmann [10] fault codewhat it means and how to fix it

Codes 10 and 18 report the outside temperature sensor, which is only fitted where the boiler is running weather compensation. On the two-character 100-W units (WB1B onwards), the 111-W, the 050-W BPJD and the Vitotronic 200-W the rows are "Short circuit, outside temperature sensor" and "Lead break, outside temperature sensor", measure "Check the outside temperature sensor and lead". The system characteristic is the reassuring part and it changes by unit: the WB1B and WB1C say "Constant mode", the B1HA "Constant temperature mode", the B1HC and BPJD "Continuous operation", and the Vitotronic 200-W "Regulates as if the outside temperature were 0 °C". All of them mean the same thing — the boiler has lost the outside reading and is heating to a fixed flow temperature instead of a compensated one, so the house will still be warm, possibly warmer than it needs to be in mild weather. No reset is listed and none is needed; the code clears when the sensor reads plausibly.

The WB1A table has no 10 or 18 because that unit does not support an outside sensor. Viessmann's repair chapter gives the contractor a resistance curve (an NTC 10 kΩ on the B1HC) and the terminal numbers to check at the control unit; the sensor lives on an outside wall, usually north-facing, and its two-core cable runs back to the boiler, which is where most lead breaks happen. There is no householder step in the operating instructions beyond noting the code. On the 2020-on numeric-table units the pair is F.13 (lead break) and F.14 (short circuit), with the same "regulates as if the outside temperature were 0 °C" behaviour.