BoilerFaultCodesVaillant P.4 Fault Code
Also written as P4 — same fault.
The Vaillant P.4 fault code means: your boiler is currently running a safety program to remove trapped air from the internal pipes and heat exchanger.. It appears on 13 Vaillant models in our database.
Technical description: Not available
What causes the Vaillant P.4 fault?
This occurs when the boiler detects potential air pockets within the internal pipework or heat exchanger, often following a recent repair or after the system has been refilled. To prevent damage from overheating, the boiler automatically runs a five-minute venting cycle to push any trapped bubbles out through the automatic air vent. It is a protective measure rather than a breakdown, ensuring the system is primed and safe to fire up.
This is more common in autumn when homeowners top up their system pressure or bleed radiators for the first time after the summer break.
DIY fix possible
Wait for the program to finish, as it usually stops automatically after 5 to 10 minutesEnsure all radiator valves are fully open to help move air through the systemCheck the boiler pressure gauge and top it up to 1.5 bar if it has dropped
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Gas Safe Register →Vaillant models that show the P.4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant P.4 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