High Lockout
F.71 on Vaillant ecoTEC

Your boiler has detected that a temperature sensor is stuck or faulty, meaning it cannot accurately monitor the water temperature to operate safely.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Flow NTC reports constant value (stuck at) | Flow NTC is defective

What causes this fault?

This fault occurs when the boiler's internal computer notices that the flow temperature sensor is sending the exact same reading regardless of whether the burner is on or off. The most common reason is that the sensor has suffered an internal electrical failure or has become covered in limescale, preventing it from detecting actual changes in water temperature.

Engineer required

This is a high severity fault. Stop using the boiler and call 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.72View →F.20View →F.27View →

Related guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →