High
White reset neon illuminated on Alpha 500e

Your boiler has sensed a lack of water flow, which is preventing it from heating your home 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.

What the manual says

Manufacturer manual

Differential flow switch operated or secondary water fault (Section J)

From this model's manual
Verified against the Alpha Boiler 500e installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

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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