4# Low

Worcester Bosch 4# Fault Code

Also written as 4 — same fault.

The Worcester Bosch 4# fault code means: your boiler is struggling to detect the outdoor temperature, which may cause your heating to fluctuate or run inefficiently.. 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: External air sensor
Typical repair cost £120-200 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Worcester Bosch models that show the 4# fault code

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

Source

Worcester Bosch 4# 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 7 July 2026