d / U Low

Pro d / U Fault Code

Also written as DU — same fault.

The Pro d / U fault code means: your boiler is undergoing a routine self-test of its internal valve to ensure it doesn't get stuck and is not actually a fault code.. It appears on 1 Pro model in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: The display will alternate between “d” and “U” and the actuator will move into the mid-position.
Typical repair cost £0 (including labour)

DIY fix possible

Wait approximately 30 seconds for the sequence to completeMonitor the display until it returns to a standard temperature readingNo action is required as this is part of the boiler's normal maintenance cycle

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Pro models that show the d / U fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Pro d / U fault code information on this page is sourced and cross-checked against the official Pro service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026