4-A2 Low

Worcester Bosch 4-A2 Fault Code

Also written as 4A2 — same fault.

The Worcester Bosch 4-A2 fault code means: your boiler is performing a routine initial setup or maintenance check to fill the internal condensate trap with water and is not actually at fault.. It appears on 1 Worcester Bosch 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: Siphon filling program: 0: switched off, 1: switched on with minimum power, 2: switched on with minimum heat output. The ++ symbol and flow temperature appear alternately in the display.

DIY fix possible

Wait approximately 15 minutes for the program to complete automaticallyEnsure your thermostat is calling for heat to allow the boiler to cycle through the processIf the code persists for over 30 minutes, restart the boiler by pressing the reset button

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Worcester Bosch models that show the 4-A2 fault code

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

Source

Worcester Bosch 4-A2 fault code information on this page is sourced and cross-checked against the official Worcester Bosch service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026