Medium
S.46 on Vaillant ecoTEC Pro

Your boiler has noticed an unstable flame when running at its lowest setting and is operating at a restricted level to prevent a complete breakdown.

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.

What the manual says

Manufacturer manual

Comfort protection: Minimum load, loss of flame. Comfort protection mode for flame loss at minimum load is activated.

Shared across the range
This code is shared by the boilers in this range, which use the same control board. It may not be printed in this exact variant’s manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

What causes this fault?

This fault occurs when the boiler struggles to maintain a steady flame while running at its lowest power level. The most common cause is a dirty or worn-out sensing probe that can no longer accurately detect the fire, or an incorrect mix of gas and air preventing a consistent burn.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Avoid the next call-out bill

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