BoilerFaultCodesViessmann 90:128 Fault Code
Also written as 90128 — same fault.
The Viessmann 90:128 fault code means: your boiler's outside temperature sensor has failed or is giving an incorrect reading, meaning the system may not heat your home efficiently based on the weather.. It appears on 5 Viessmann models in our database.
Technical description: Time constant for calculating the adjusted outside temperature 21.3 hours
What causes the Viessmann 90:128 fault?
This fault occurs when the boiler receives inconsistent or 'out of range' data from the sensor mounted on the exterior of your property. The most common reason is a hardware failure within the sensor itself or moisture ingress into the sensor's casing that disrupts the electrical signal. It can also be caused by a loose connection or a break in the thin wiring that links the sensor to the boiler's internal control board.
This issue often surfaces during autumn or winter when heavy rain and frost are more likely to affect external wiring and sensor enclosures.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Viessmann models that show the 90:128 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann 90:128 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