Low Warning
D.17 on Vaillant ecoTEC

Your boiler has switched its control method to the return pipe because it is detecting a potential issue with the main flow sensor.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description

Heating flow return regulation

What causes this fault?

This fault occurs when the boiler detects a logical error or an inconsistent reading from the flow temperature sensor. To keep your heating running safely, the boiler automatically switches to monitoring the return pipe sensor while it ignores the potentially faulty flow data. The most common reason is a failing or 'drifting' sensor that is sending incorrect electrical signals back to the main control board.

Engineer required

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

Find a Gas Safe engineer
Search the official register →

Related fault codes

F.22View →F.23View →F.24View →

Related guides you might find useful

No hot water but heating works

Almost always a stuck diverter valve on combis.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →