BoilerFaultCodesViessmann F39 Fault Code
The Viessmann F39 fault code means: your boiler has lost connection with its indoor thermostat, meaning it cannot correctly measure the temperature inside your home to control the heating.. It appears on 1 Viessmann model in our database.
Technical description: Lead break, room temperature sensor
DIY fix possible
Check if your wireless thermostat has run out of batteries and replace them if necessaryEnsure the thermostat is within range of the boiler and has not been moved too far awayCheck for any loose wiring if you have a visible wired wall thermostat
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Avoid the next call-out bill
A cover plan would handle faults like F39: no call-out fees, no surprise bills.
Affiliate links. We may earn a commission at no extra cost to you.
Viessmann models that show the F39 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann F39 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 7 July 2026