BoilerFaultCodesWorcester Bosch A4 Fault Code
The Worcester Bosch A4 fault code means: your boiler's hot water sensor is struggling to communicate with the main control board, meaning the system cannot accurately monitor the temperature of your hot water.. It appears on 5 Worcester Bosch models in our database.
Technical description: DHW sensor fault. DHW sensor 10000 - 12000 ohms at room temperature. Wiring and connections to sensor.
What causes the Worcester Bosch A4 fault?
This fault occurs when the sensor responsible for monitoring your hot water temperature fails to send a clear signal to the boiler's internal computer. The most common reason is that the sensor has physically degraded over time or a small leak has caused corrosion on the electrical connections. Without this data, the boiler stops heating water as a safety precaution to prevent it from becoming too hot.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Worcester Bosch models that show the A4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch A4 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