BoilerFaultCodesVaillant P.5 Fault Code
Also written as P5 — same fault.
The Vaillant P.5 fault code means: your boiler is running a specialized internal test to ensure its safety sensor correctly shuts the system down if it gets too hot.. It appears on 17 Vaillant models in our database.
Technical description: Test program for the safety temperature limit check; appliance heats up by by-passing a regulating shut down through the flow regulator until the shut off temperature of 97 °C is achieved.
What causes the Vaillant P.5 fault?
This is not a mechanical failure, but rather a deliberate test mode triggered by a heating engineer during a service. It occurs when the boiler is commanded to bypass its normal temperature controls to verify that the safety sensors will correctly shut the system down if it reaches a high heat threshold. Once the boiler reaches the target temperature and confirms the safety limit works, the test concludes and the boiler returns to normal operation.
DIY fix possible
Wait for the test to complete automaticallyDo not turn the boiler off during the processIf the code persists, restart the boiler using the reset button
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Vaillant models that show the P.5 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant P.5 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
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Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026