BoilerFaultCodesWorcester Bosch 1-b4 Fault Code
Also written as 1B4 — same fault.
The Worcester Bosch 1-b4 fault code means: your boiler is struggling to accurately measure the temperature of the water going to your taps, which might result in the water being too hot or too cold.. It appears on 1 Worcester Bosch model in our database.
Technical description: Hot water temperature outlet: Hot water temperature.
What causes the Worcester Bosch 1-b4 fault?
This fault occurs when the boiler's internal computer receives an unconventional or missing signal from the sensor that monitors your tap water temperature. The most common reason is that the sensor has either developed a physical fault or has been affected by a small amount of corrosion on its electrical connections. In some cases, localized limescale buildup around the sensor can also cause it to send inaccurate readings.
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 1-b4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch 1-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