High Lockout
E31 on Ravenheat CSI 85

Your boiler has lost its digital connection with your external smart thermostat or heating controls, meaning it cannot receive instructions to turn on.

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

OpenTherm communication error

What causes this fault?

This fault occurs when the digital 'conversation' between your boiler and your smart thermostat is interrupted, leaving the boiler without instructions. The most common reasons are a loose connection in the control wiring, a software glitch in the thermostat unit, or a complete failure of the communication module inside the boiler.

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

E29View →

Related guides you might find useful

Boiler won't turn on — 9-step checklist

Work through these before the call-out.

Read →

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 →