How much does it cost to fix Vaillant error code S.34?

Typical repair cost for the Vaillant S.34 fault is £90-180 including labour.

S.34 High

Vaillant S.34 Fault Code

Q: What does the Vaillant S.34 fault code mean?

Your boiler has paused operation because it believes the internal temperature is so low that the pipes are at risk of freezing.

Also written as S34 — same fault.

The Vaillant S.34 fault code means: your boiler has paused operation because it believes the internal temperature is so low that the pipes are at risk of freezing.. It appears on 42 Vaillant models in our database.

Technical description: Heating mode frost protection error

What causes the Vaillant S.34 fault?

This fault occurs when the boiler's internal sensors detect that the water temperature has dropped near freezing levels, triggering a safety pause to prevent pipe damage. The most common reason is that the boiler is located in a very cold area, like a garage or loft, and the frost protection system has been activated to protect the heat exchanger. It can also be triggered by a faulty temperature sensor sending an incorrect freezing signal to the main control board.

This fault is almost exclusively seen during periods of extreme winter weather or prolonged cold spells when external temperatures drop below freezing.

Typical repair cost £90-180 (including labour)

DIY fix possible

Check that the room thermostat and radiator valves are turned upEnsure the boiler's power supply is constant and has not been switched offIf the condensate pipe is outside, check if it is frozen and gently thaw it with warm waterWait 15-30 minutes for the boiler to warm up naturally before resetting

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Vaillant models that show the S.34 fault code

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

Source

Vaillant S.34 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