Viessmann [30] fault codewhat it means and how to fix it

30 and 38 are a pair. On every two-character Vitodens control unit — WB1A, WB1B, WB1C, B1HA/B1KA, B1HC/B1KC, the 111-W B1LA to B1LD and the 050-W BPJD — and on the Vitotronic-controlled 200-W, 222-F and 242-F, the rows read "Burner blocked — Short circuit, boiler water temperature sensor" for 30 and "Burner blocked — Lead break, boiler water temperature sensor" for 38, with the single measure "Check the boiler water temperature sensor". The sensor is an NTC 10 kΩ thermistor immersed in the flow from the heat exchanger, and it is the temperature the control unit modulates the burner against; a reading of zero ohms or infinite ohms is not a temperature, so the burner is blocked rather than run blind. Blocked means exactly that: no reset is listed, and the boiler resumes by itself if the sensor comes back — which it will, briefly, when a lead with a break in it happens to make contact.

Viessmann's repair chapter gives the contractor a resistance curve to check the sensor against, and on the WB1A warns that the sensor is immersed in the heating water and the boiler must be drained on the primary side before it is changed — a note that explains why a small part carries a real labour charge. There is no householder action in any operating instructions we read for 30 or 38 beyond noting the code. The neighbouring code 52/5A is the DHW flow sensor, 50/58 the cylinder sensor, 51/59 the DHW outlet sensor; 30/38 is the boiler itself. On the 2020-on numeric-table units the equivalents are F.184 (short circuit) and F.185 (lead break), where the same sensor doubles as the high limit safety cut-out, and F.446 for a deviation between the two readings.