On / Off (Demand fault) Medium

Worcester Bosch On / Off (Demand fault) Fault Code

Also written as ONOFFDEMANDFAULT — same fault.

The Worcester Bosch On / Off (Demand fault) fault code means: your boiler is receiving a signal to turn on even though your thermostat or timer is not asking for heating or hot water, which may keep the boiler running unnecessarily.. 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: Boiler operating without live demand at ST10-LR (from external roomstat timer). Check for low voltage return at ST10-LR from older thermostats or permanent live from another source.
Typical repair cost £120-250 (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 On / Off (Demand fault) fault code

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

Source

Worcester Bosch On / Off (Demand fault) 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