High Lockout
L05 on Ideal Logic Max Combi

Your boiler has lost its ability to accurately measure the temperature of the water coming back into the system, which is 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 Fault

What causes this fault?

This fault occurs when the sensor responsible for monitoring the temperature of the water returning to the boiler fails or provides an illogical reading. The most common reason is that the internal component has suffered an electrical failure or the wiring connecting it to the main control board has become loose or damaged.

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 →