Medium
D4 on Worcester Bosch Greenstar 12i System

Your boiler has detected that your radiators are not transferring heat properly, causing the water returning to the unit to be much cooler than the water leaving it.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Temperature difference between supply and return exceeding limit

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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