High Lockout
E25 on Ravenheat CSI 120

Your boiler has detected that the water inside the main heating unit has frozen due to extremely cold temperatures, preventing it from operating.

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

Main heat exchanger frozen

What causes this fault?

This fault occurs when the temperature sensors inside the boiler detect that the water within the heat exchanger has dropped low enough to freeze. The most common reason is that the boiler is located in an unheated area, such as a garage or loft, without adequate insulation or frost protection during a severe cold snap.

This fault is exclusively seen during the winter months when external temperatures drop below freezing for extended periods.

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

E02View →

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