High Lockout
F 04 OR L 04 on Ideal Logic Max Combi

Your boiler has detected a problem with the sensor that monitors the temperature of the water leaving the unit, meaning it cannot safely control the heat.

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 internal computer stops receiving a reliable signal from the sensor that monitors the temperature of the water being sent to your taps or radiators. The most common reason is that the sensor itself has developed a physical defect or the electrical wiring connecting it to the main control board has become damaged or loose. Without this data, the boiler shuts down as a safety precaution to prevents it from potentially 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

F1View →L1View →F2View →L2View →

Related guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →