The most common boiler faults are low pressure (below 1 bar), fault codes from the display, a kettle-like rumbling noise (kettling from limescale), the pilot light going out, and the boiler switching itself off. Most require a Gas Safe engineer — never attempt boiler repairs yourself. Low pressure can sometimes be fixed by repressurising via the filling loop.
Why Is My Boiler Showing a Fault Code?
Modern boilers are equipped with self-diagnostic systems that display error codes when something goes wrong. These codes vary by manufacturer, but they all signal the same thing: the boiler has detected a fault and locked out to protect itself and your home. A single reset is often fine after a brief power interruption — but a boiler that keeps locking out is telling you something structural needs fixing.
Never ignore recurring fault codes and simply keep resetting. Doing so doesn't fix the problem — it delays the fix while the underlying fault worsens.
Fault: Low Boiler Pressure (Most Common)
Your boiler's pressure gauge should read between 1.0 and 1.5 bar when the system is cold. If it drops below 1 bar, the boiler will often lock out automatically. According to Worcester Bosch, low pressure accounts for approximately 25% of all boiler service call-outs in the UK.
You can repressurise a combi boiler yourself via the filling loop — the flexible connector linking the cold mains to the heating circuit. However, if your pressure drops repeatedly within weeks of repressurising, you have a leak somewhere in the system that needs locating and fixing.
Fault: Boiler Kettling (Rumbling or Banging Noise)
Kettling — a rumbling or boiling sound from inside the boiler — is caused by limescale or sludge restricting water flow in the heat exchanger, causing localised boiling. It's more common in hard-water areas like West Yorkshire. Left untreated, kettling damages the heat exchanger, which is the most expensive component to replace (£400–£800 on most boilers).
The fix is a system power flush followed by a chemical inhibitor dose to prevent future scale build-up.
Fault: Pilot Light Going Out
If your boiler has a pilot light that keeps extinguishing, the thermocouple — a safety device that detects whether the flame is lit — is almost certainly faulty. This is a straightforward repair but must be carried out by a Gas Safe registered engineer. Never attempt to repair gas components yourself.
Fault: Boiler Switches Itself Off (Short-Cycling)
A boiler that turns on then shuts off again rapidly is short-cycling. It wastes energy, increases wear, and usually points to one of these causes:
- Low water pressure — the most common cause
- Faulty thermostat — sending incorrect temperature signals to the boiler
- Blocked or frozen condensate pipe — especially in winter; this pipe carries waste water from the boiler to a drain
- Closed or partially closed valve — reducing flow through the system
- Oversized boiler — a boiler too large for the heating system will reach target temperature too quickly and shut off repeatedly
Fault: No Hot Water but Heating Works
On a combi boiler, this is the classic symptom of a failed diverter valve. The diverter valve is the component that switches between heating mode and hot water mode. When it sticks in the heating position, your radiators heat up but your taps run cold. A diverter valve replacement typically costs £150–£300 including parts and labour on a standard combi boiler.
Fault: Water Leaking From the Boiler
Water around or beneath the boiler casing is serious. Possible causes include a failed pressure relief valve, a corroded heat exchanger, loose pump seals, or damaged pipe fittings. Turn the boiler off, shut off the water supply, and call a Gas Safe engineer immediately. Do not attempt to open the boiler casing yourself — gas appliances must only be worked on by registered professionals.
Warning: Yellow or Orange Flame
A healthy gas flame is blue. A yellow or orange flame, or black soot marks around the boiler casing, indicates incomplete combustion and a potential carbon monoxide risk. Carbon monoxide is odourless and colourless — it kills approximately 60 people in the UK each year. If you see these signs: leave the property immediately, open windows on the way out, and call the Gas Emergency line on 0800 111 999.
When to Repair vs Replace Your Boiler
A boiler under 10 years old is almost always worth repairing — typical repair costs of £150–£350 are a fraction of replacement. For boilers between 10 and 15 years old, weigh the repair cost against a new installation. Boilers over 15 years old, or those requiring a repair costing more than 50% of a replacement boiler, are almost always better replaced with a modern A-rated unit.
Gastech 24/7 Plumbing & Heating Services
Gas Safe registered engineers serving Keighley & West Yorkshire since 2012
