What to check, in order
- Check the pressure gauge
Low system pressure is one of the causes Worcester lists, and its manual says the boiler may stop below 1 bar.
- If it's low, top up and reset
Fill to 1.5 bar with the filling link, then hold the reset button for 3 seconds.
- Otherwise, leave it to an engineer
If the pressure was fine, or E9 comes back, don't try to put it right yourself. Worcester's manual says to contact a Gas Safe registered engineer.
When to call a Gas Safe engineer
Worcester's service booklet lists a seized pump, a faulty flow or flue safety limiter, a wiring fault, a blown fuse and a faulty control board among the causes.
A repair visit will usually check the pump, the flow of water round the system and the overheat thermostat.
Before you book
- Note the model and serial number. They're on a label on the data badge, usually under the boiler or behind the front cover.
- Write down the exact code, and what the appliance was doing when it appeared.
- Check whether it's still under warranty. If it is, the repair may cost you nothing.
Good to know
- On an open-vented system, with a feed and expansion tank in the loft, there's no pressure gauge to check. Worcester's engineers check the tank has enough water in it.
Checked against
- Worcester Bosch Greenstar CDi Classic user instructions (PDF)
- Worcester Bosch Greenstar CDi Classic engineer's service booklet (PDF)
Last reviewed September 2026. Manufacturers update their pages, and your manual is the final word.