What to check, in order
- Check the condensate pipe outside
Worcester says a frozen or blocked condensate pipe shuts the boiler down. In freezing weather, look along the outside section.
- Thaw it gently
Hold a hot water bottle, a microwaveable heat pack or a cloth soaked in hot water against the frozen part. Don't use boiling water, and only do this if the pipe is at ground level and easy to reach.
- Reset and wait
Hold the reset button for 5 seconds, then wait two to three minutes for the boiler to restart.
When to call a Gas Safe engineer
If d5 stays after the pipe is clear, call a Gas Safe registered engineer to look at the condensate pump.
Good to know
- This meaning is from Worcester's manual for the Greenstar FS CDi. A code can mean something different on another range, so check the manual for your model.
Parts people end up buying
Only if the checks above point to a part. Match it to your model number first: it's on a label on the data badge, usually under the boiler or behind the front cover.
- Condensate pipe insulationSearch on Amazon
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Checked against
- Worcester Bosch Greenstar FS CDi user instructions (PDF)
- Worcester Bosch Greenstar CDi Classic user instructions (PDF)
Last reviewed September 2026. Manufacturers update their pages, and your manual is the final word.