1-d4 Low

Worcester Bosch 1-d4 Fault Code

Also written as 1D4 — same fault.

The Worcester Bosch 1-d4 fault code means: your boiler is reporting a communication problem or a technical fault with your solar heating system, though your central heating should continue to work.. It appears on 8 Worcester Bosch models 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: Solar fault: Fault code (Is only displayed, if a solar module is connected)

What causes the Worcester Bosch 1-d4 fault?

This fault occurs when the boiler's main control board loses communication with the solar thermal control module. The most common reason is a loose connection in the wiring between the units or a localized power failure affecting the solar components. It can also be triggered if the solar temperature sensor provides an unrealistic reading due to a component failure.

This fault is more common during the summer months when solar thermal systems are under higher demand and temperature sensors are pushed to their limits.

Typical repair cost £90-250 (including labour)

DIY fix possible

Locate the solar control module or pump station.Check if any cables connecting the solar system to the boiler have come loose.Restart the boiler using the reset button on the display.

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Worcester Bosch models that show the 1-d4 fault code

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

Source

Worcester Bosch 1-d4 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