Flashing mains LED High

Worcester Bosch Flashing mains LED Fault Code

Also written as FLASHINGMAINSLED — same fault.

The Worcester Bosch Flashing mains LED fault code means: your boiler has detected an internal sensor failure and has stopped working to prevent damage or overheating.. It appears on 1 Worcester Bosch model in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Indicates a fault condition; check thermistor resistance (690ohms to 33k) on pins 3 & 4 or pins 19 & 20 to determine if there is an open or short circuit.

What causes the Worcester Bosch Flashing mains LED fault?

This fault occurs when the boiler's internal sensors, which monitor water temperature, stop sending a reliable signal to the main control board. The most common reason is that the sensor has physically degraded or developed an electrical short, meaning the boiler can no longer safely regulate its heat output. In some cases, old or damaged wiring between the sensor and the brain of the boiler is the culprit.

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 Flashing mains LED fault code

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

Source

Worcester Bosch Flashing mains LED 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