S.98 Low

Vaillant S.98 Fault Code

Also written as S98 — same fault.

The Vaillant S.98 fault code means: your boiler is temporarily pausing because it has noticed an unusual temperature difference between two internal sensors and is performing a self-check to ensure everything is working correctly.. It appears on 25 Vaillant 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: Flow return sensor check is running demand is block

What causes the Vaillant S.98 fault?

This status occurs when the boiler's internal computer notices that the temperatures on the flow and return pipes are not changing in the way it expects. To prevent damage, the boiler pauses operation to double-check that the sensors are reporting accurately and that water is actually moving through the heat exchanger. This is often caused by a failing pump or a buildup of debris that is slowing down the flow of water.

This issue is more common during the first cold snap of autumn when a pump that has been idle all summer fails to restart correctly.

Typical repair cost £90-180 (including labour)

DIY fix possible

Check that all radiator valves are fully openEnsure your central heating system pressure is between 1.0 and 1.5 barWait up to 15 minutes for the boiler to complete its automatic recalibration

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →

Vaillant models that show the S.98 fault code

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

Source

Vaillant S.98 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026