High Lockout
L4 on Ideal Logic Max Combi

Your boiler has sensed a problem with the internal thermometer that monitors the water temperature leaving the unit, causing it to shut down for safety.

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 Thermistor Fault

What causes this fault?

This fault occurs when the boiler's control system receives an incorrect or missing signal from the sensor that monitors hot water leaving the unit. It is usually caused by the sensor itself failing due to age, or a loose connection in the internal wiring preventing the computer from reading the temperature accurately.

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

L1View →L2View →F2View →

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 →