High Lockout
F5 on Ideal Logic Combi ESP1

Your boiler's internal sensor that monitors the temperature of the water coming back into the unit has developed a fault, preventing the system from operating 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 boiler's internal computer stops receiving a reliable signal from the sensor that monitors the temperature of the water returning from your radiators. The most common reason is that the sensor itself has worn out or suffered an electrical failure, meaning it can no longer tell the boiler if it is safe to continue heating.

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

F4View →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 →