High Lockout
F01 on Intergas Rapid 25

Your boiler has detected a fault with its internal temperature sensor, meaning it cannot safely measure the heat of the water and has stopped working as a precaution.

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

S1 sensor short circuit

What causes this fault?

This fault occurs when the boiler's control board loses its connection to the flow temperature sensor or receives an incorrect electrical signal. Most commonly, this is caused by the internal sensor failing due to age or the electrical wiring becoming damaged or loose. Because the boiler no longer knows how hot the water is, it shuts down to prevent the system from dangerously overheating.

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

F02View →F03View →

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 →