High Lockout
E135 on Potterton Promax HE

Your boiler's internal computer has encountered a software glitch or a communication failure that is preventing it from starting up safely.

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

Internal error

What causes this fault?

This fault occurs when the boiler's main control board experiences a critical internal failure or loses the ability to talk to other sensors. It is essentially a software crash where the 'brain' of the boiler can no longer verify that it is safe to operate. This is usually caused by a component failure on the circuit board itself or a loose connection in the internal wiring.

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.

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Related fault codes

E128View →E133View →E160View →

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