What to check, in order
- Check the pressure gauge
Worcester's CDi Classic manual gives 1.5 bar as the ideal, and says the boiler may stop below 1 bar.
- Top up with the filling link
Open the filling link and let water in until the gauge reads 1.5 bar, then close it.
- Reset the boiler
Hold the reset button for 3 seconds and let go. The boiler restarts.
When to call a Gas Safe engineer
If A1 comes back with the pressure right, call a Gas Safe registered engineer. Worcester's service booklet lists a seized or sticking pump, worn pump bearings and air in the system, and says to check for leaks if the pressure is below 1.2 bar.
Good to know
- This meaning is from Worcester's service booklet for the Greenstar CDi Classic. On other Greenstar ranges, check the manual for your model.
Quick answers
What does the Worcester Bosch A1 fault code mean?
The boiler's pump has been running without enough water in the system, so the boiler has locked out. Low system pressure is the first thing Worcester's engineers check.
Can I fix Worcester Bosch A1 myself?
Easy (about 15 minutes). Start with: check the pressure gauge; top up with the filling link; reset the boiler. If A1 comes back with the pressure right, call a Gas Safe registered engineer. Worcester's service booklet lists a seized or sticking pump, worn pump bearings and air in the system, and says to check for leaks if the pressure is below 1.2 bar.
Is Worcester Bosch A1 worth repairing?
Worth fixing. Often it's just low pressure. If the pressure is fine, the pump is the likely part, and replacing it is an engineer's job.
Checked against
- Worcester Bosch Greenstar CDi Classic engineer's service booklet (PDF)
- Worcester Bosch Greenstar CDi Classic user instructions (PDF)
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.