High Lockout
L5 on Ideal Logic Combi C24 C30 C35

Your boiler has detected a problem with the sensor that monitors the temperature of the water coming back into 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

Return Thermistor Fault

What causes this fault?

This fault occurs when the boiler's control board receives an impossible or inconsistent reading from the sensor that monitors water returning from your radiators. The most common reason is that the sensor has physically failed or developed a short circuit, but it can also be triggered by damaged internal wiring or a fault with the main circuit board.

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 →

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 →