90:0 to 90:199 Low

Viessmann 90:0 to 90:199 Fault Code

Also written as 900TO90199 — same fault.

The Viessmann 90:0 to 90:199 fault code means: your boiler's control system is struggling to adjust the internal water temperature correctly in response to changes in the weather outside.. It appears on 5 Viessmann models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Fast (low values) or slow (high values) matching of flow temperature subject to set value if the outside temperature changes; 1 step = 10 min

What causes the Viessmann 90:0 to 90:199 fault?

This fault occurs when the boiler's control system loses accurate data from the external sensor that tracks the weather. It is usually caused by a loose electrical connection, a damaged cable, or the sensor itself failing, which prevents the boiler from knowing how hot it needs to run to keep your home comfortable.

This issue is often first noticed in autumn or winter when the boiler needs to react more frequently to dropping outdoor temperatures.

Typical repair cost £100-£250 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Viessmann models that show the 90:0 to 90:199 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Viessmann 90:0 to 90:199 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