P.01 Medium

Vaillant P.01 Fault Code

Also written as P01 — same fault.

The Vaillant P.01 fault code means: your boiler has failed while running a specific test used for commissioning or high-power performance checks.. It appears on 29 Vaillant models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Maximum load test program error

What causes the Vaillant P.01 fault?

This fault occurs when the boiler's internal diagnostic program fails to complete its high-performance test correctly. It is usually triggered manually by an engineer during a service or installation, and the error suggests that the system couldn't maintain the required heat output to finish the check. The most common reason is a sensor reading an unexpected temperature or the test being interrupted before the sequence was finished.

Typical repair cost £80-160 (including labour)

DIY fix possible

Turn the boiler power off at the wall or fused spurWait 60 secondsTurn the power back on and press the 'Reset' button (indicated by a flame with a line through it)

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Vaillant models that show the P.01 fault code

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

Source

Vaillant P.01 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 31 August 2026