High Lockout
F05 L05 on Ideal Logic Max Combi

Your boiler has detected a problem with the sensor that monitors the temperature of the water coming back from your radiators, preventing it from heating up correctly.

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

Return Thermistor

What causes this fault?

This fault occurs when the boiler's internal computer receives an incorrect or missing signal from the sensor that monitors the water returning from your radiators. The most common reason is that the sensor has physically failed due to age or electrical wear, though it can also be caused by a loose connection or damaged wiring inside the unit.

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 →L02View →

Related guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Radiators cold at the bottom

Sludge build-up — needs flushing, not bleeding.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →