High Lockout
0E99 on Alpha Pro Tec Plus

Your boiler is struggling to talk to another heating unit connected to it, which is 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

Simple cascade communication error.

What causes this fault?

This fault occurs when the communication link between multiple boilers in a shared system is broken or interrupted. The most common reason is a loose connection in the wiring that links the units together, or a failure in the electronic control board that manages how the boilers talk to each other.

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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