A gas boiler usually gives notice before it fails completely. None of the signs below are a reason to take the case off the appliance or to work on gas parts yourself. They are a reason to call a Gas Safe registered engineer.
The boiler will not stay on
Repeated lockouts, a fault code that returns after a reset, or a boiler that fires briefly then stops, all point to a fault that needs diagnosis. Resetting once to restore heat is reasonable. Resetting through an evening is not a repair.
No heating, no hot water, or both
If the house is cold but the taps are still hot, the fault may be on the heating side of the system rather than a total boiler failure. If both have gone, the appliance itself is the first place to look. Either way, the useful information to have ready is which of the two has failed and whether any code is showing.
See boiler repairs for how we approach that visit.
Pressure that will not hold
A boiler that keeps losing pressure, or that will not run because the pressure is too low, has a cause somewhere on the appliance or the heating circuit. Topping it up once can be a short-term step if you already know how the filling loop on your boiler works. Repeated filling is a symptom, not a solution.
Do not guess at leaks behind the boiler or on hidden pipework.
Noise, kettling or radiators that stay cold
Kettling, banging or radiators that remain cold while the boiler is firing can come from circulation, sludge, valves or the appliance. They are worth inspecting before the next cold spell, particularly if they have become worse over a winter.
When the next conversation is replacement
Age, repeated major parts and an appliance that no longer matches how the house is used all push the decision towards repair or replace. That decision is clearer after an inspection than from a fault code alone.
If you smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999.