The Worcester Greenstar Cdi is a mains gas combi / regular boiler. Below are the faults it most often develops, what the error codes mean, and how to get it fixed at a fixed price by a Gas Safe registered engineer.
Variants covered: 25 Cdi, 30 CDi, 35 CDi, 40 Cdi, 40 Cdi Conventional, 30 Cdi Conventional, 30 Cdi System, 27 CDi, 31 CDi
Hot taps run cold while heating still works — commonly the diverter valve, the flow sensor or the plate heat exchanger scaling up.
The gauge drops below ~1 bar and the boiler cuts out — usually a small leak, a tired expansion vessel or a weeping pressure-relief valve.
It tries to light, fails and locks out — often the spark electrode, gas valve or a flame-sensing fault.
Drips or staining under the case — a seal, the pump, the heat exchanger or a corroded pipe.
Rumbling on the heat exchanger, usually limescale or sludge restricting flow.
Cool rads or slow warm-up — often the pump, a sticking motorised valve or air in the system.
These are the likely causes — a Gas Safe registered engineer confirms the fault on site before any work. If you can smell gas, stop and ring 0800 111 999.
If there's a code on the display, tap it for the meaning, the free checks to try first and a fixed repair price:
One fixed price, no callout fee, a vetted Gas Safe engineer who covers your postcode.