The Baxi ASSURE 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 COMBI, 30 COMBI, 36 COMBI, 12 SYSTEM, 15 SYSTEM, 18 SYSTEM, 24 SYSTEM, 13 HEAT, 16 HEAT, 19 HEAT, 25 HEAT, 30 HEAT
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.