BoilerFaultCodesViessmann 67:40 Fault Code
Also written as 6740 — same fault.
The Viessmann 67:40 fault code means: your boiler's solar heating accessory has detected a minor internal setting mismatch or a sensor communication issue regarding your hot water targets.. It appears on 5 Viessmann models in our database.
Technical description: In conjunction with Vitosolic solar control unit: Set DHW value 3
What causes the Viessmann 67:40 fault?
This fault happens because the main boiler computer has lost its digital connection with the solar heating control panel. It is usually caused by a loose communication cable between the two units or a electronic failure within the solar module itself. When this link breaks, the boiler doesn't know how much heat the solar panels are providing, which can lead to inefficient water heating.
This issue is often more noticeable during the spring and summer months when homeowners expect their solar panels to handle the majority of their hot water production.
DIY fix possible
Locate the Vitosolic solar control unit and check for any separate power switches.Perform a standard reset of the boiler's control panel by pressing the reset button.Check that the solar cylinders temperature settings have not been adjusted to an unusually high or low level.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Viessmann models that show the 67:40 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 67:40 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026