LED 4 flash/second Medium

Worcester Bosch LED 4 flash/second Fault Code

Also written as LED4FLASHSECOND — same fault.

The Worcester Bosch LED 4 flash/second fault code means: your boiler has detected an issue with the sensor that manages your hot water, meaning you will likely only have heating or very inconsistent water temperatures.. It appears on 1 Worcester Bosch model in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: DHW sensor failure during No DHW function (CH OK)

What causes the Worcester Bosch LED 4 flash/second fault?

This fault occurs when the internal sensor responsible for measuring the temperature of your hot water fails or sends an incorrect signal to the main control board. The most common reason is that the sensor has physically degraded over time or developed a wiring fault, preventing the boiler from knowing how hot the water actually is. As a safety precaution, the boiler will often stop providing hot water to prevent accidental scalding.

Typical repair cost £90-160 (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 LED 4 flash/second fault code

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

Source

Worcester Bosch LED 4 flash/second 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