BoilerFaultCodesWorcester Bosch A-b4 Fault Code
Also written as AB4 — same fault.
The Worcester Bosch A-b4 fault code means: your boiler has detected an issue with the sensor that monitors the temperature of the hot water leaving the unit, which may result in inconsistent water temperatures.. It appears on 1 Worcester Bosch model in our database.
Technical description: Hot water temperature: Outlet temperature
What causes the Worcester Bosch A-b4 fault?
This fault occurs when the boiler's internal computer receives an implausible or missing signal from the sensor that monitors your hot water. The most common reason is that the sensor has developed an electrical fault or has become corroded, preventing it from accurately telling the boiler how hot the water is. In some cases, the issue may be caused by loose or damaged wiring connecting that sensor to the main control board.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Worcester Bosch models that show the A-b4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch A-b4 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