BoilerFaultCodesViessmann 9F:0 to 9F:40 Fault Code
Also written as 9F0TO9F40 — same fault.
The Viessmann 9F:0 to 9F:40 fault code means: your boiler is notifying you that there is a specific temperature difference between the water flowing out and returning, which usually indicates an adjustment is needed in the system's settings or water flow.. It appears on 5 Viessmann models in our database.
Technical description: Differential temperature adjustable from 0 to 40K
What causes the Viessmann 9F:0 to 9F:40 fault?
This fault happens when the difference in temperature between the water leaving the boiler and the water returning to it exceeds the limit you have manually set in the settings. It usually occurs because water isn't moving through the system at the expected speed, often due to a pump working too slowly or a slight blockage in the pipes. In some cases, it simply means the monitoring sensors are giving slightly inaccurate readings and need to be recalibrated.
This code is more common when the heating is first turned on in autumn, as pumps can seize after being idle all summer or air trapped in the radiators can restrict the flow of water.
DIY fix possible
Check that all radiator valves are fully openEnsure the central heating system pressure is between 1.0 and 1.5 barBleed any air from radiators to ensure smooth water flow
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Gas Safe Register →Viessmann models that show the 9F:0 to 9F:40 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 9F:0 to 9F: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