High
Phase reversed neon (behind the knob) lighting on Malvern TenTwenty

Your boiler has detected that the incoming electrical wires are connected the wrong way around, preventing it from safely starting the main burner.

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

L and N supply are reversed; the boiler will not progress beyond pilot ignition.

From this model's manual
Verified against the Malvern Tentwenty 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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