High Lockout
F.00 on Vaillant Thermocompact

Your boiler has stopped working because it can no longer accurately measure the temperature of the water leaving the unit due to a sensor or wiring fault.

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: - NTC broken - NTC cable broken - Defective connection at NTC - Defective connection at electronics | Flow-NTC cable defective/broken NTC faulty

What causes this fault?

This fault occurs when the boiler's main control board loses communication with the sensor that monitors the temperature of the water exiting the unit. The most common reason is that the internal sensor has developed an electrical failure or the thin wiring connecting it to the main circuit board has become loose or damaged.

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.01View →F.10View →F.11View →

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 →