Most boiler breakdowns come down to a handful of causes: low water pressure, ignition failure, a frozen condensate pipe, a faulty sensor, or a worn part such as a pump or diverter valve. Some of these you can resolve yourself in a few minutes; others need a Gas Safe registered engineer. This guide explains the common faults, what the codes mean, and how to tell the difference.
What usually goes wrong
Boilers are reliable, but they have several points that fail more often than others. Knowing the usual suspects helps you describe the problem accurately and decide whether it is urgent.
- Low pressure — the water pressure in the system has dropped below the level the boiler needs to run. Common after a radiator bleed or a small leak somewhere in the pipework.
- Ignition failure — the boiler tries to fire but cannot light or stay lit. This can be a gas supply issue, a faulty ignition lead, or a blocked condensate pipe.
- No hot water but heating works (or vice versa) — often a stuck diverter valve, the part that directs hot water between your taps and your radiators.
- Frozen condensate pipe — common in cold snaps. The pipe that carries waste water outside freezes and backs up, shutting the boiler down.
- Strange noises — banging, gurgling or whistling (often called "kettling") usually points to trapped air, low pressure, or limescale on the heat exchanger.
- Leaking — water around the boiler can mean a corroded seal, a failing pump, or a pressure problem. A persistent leak should never be ignored.
- Radiators cold at the bottom or top — sludge build-up or trapped air rather than a boiler fault, but it affects performance.
Anything involving the smell of gas is different. If you suspect a gas leak, turn off the supply at the meter, open windows, avoid switches and naked flames, and call the National Gas Emergency line on 0800 111 999.
Reading common fault codes
Some of these you can resolve yourself in a few minutes; others need a Gas Safe registered engineer.
Modern boilers show a fault code on the display when something stops working. The code is a clue, not a diagnosis — the same code can have several causes. Always check your own manual, because codes differ between manufacturers.
That said, some patterns are widely shared across brands:
- Low pressure codes — many boilers flag this when the gauge reads below roughly 1 bar. The display may show a code or a pressure symbol. This is often fixable yourself.
- Ignition or flame failure codes — indicate the boiler could not detect a flame. Causes range from a frozen condensate pipe to a faulty electrode or an interrupted gas supply.
- Fan or air pressure codes — suggest the fan or its sensor has a problem, which prevents safe combustion. This needs an engineer.
- Sensor or thermistor codes — point to a temperature sensor reading incorrectly. Usually a parts replacement.
- Overheat or lockout codes — the boiler has shut itself down as a safety measure. A reset may clear it, but if it recurs, leave it off and get it checked.
If a code clears after a reset and does not return, the fault may have been a one-off. If the same code reappears, note it down with the boiler make and model before arranging a callout — it speeds up the visit.
Quick checks before calling out
Several common faults have simple fixes that do not need an engineer. Working through these first can save a callout fee.
- Check the pressure gauge. If it reads below 1 bar, the system is low. Most boilers have a filling loop — a small valve or braided hose underneath — that lets you top it back up to around 1 to 1.5 bar. The manual shows where it is.
- Look at the thermostat and timer. A flat battery in a wireless thermostat or an incorrect time setting can stop heating from coming on. Replace the battery and confirm the schedule.
- Check the power and the gas. Make sure the boiler has power and that other gas appliances work. If your cooker is out too, the issue may be the supply, not the boiler.
- Thaw a frozen condensate pipe. In freezing weather, pour warm — not boiling — water over the white plastic pipe running outside, or hold a wrapped hot water bottle against it. Once thawed, reset the boiler.
- Try one reset. Most boilers have a reset button. Press it once and wait. If the fault returns immediately, stop — repeated resets can mask a real problem.
- Bleed the radiators. Cold patches at the top suggest trapped air. Bleeding can restore performance, though it may lower system pressure, so check the gauge afterwards.
If these steps do not work, or the fault involves the fan, the gas valve, the heat exchanger, or anything inside the casing, stop there. By law, work on the gas side of a boiler must be done by a Gas Safe registered engineer. You can confirm an engineer's registration on the Gas Safe Register website.
When a repair beats replacement
A repair usually makes sense when the boiler is relatively young, the fault is a single failed part, and the unit is otherwise efficient. Replacing a pump, sensor, or diverter valve on a boiler under about eight years old is often the sensible choice.
Replacement starts to look better when repairs become frequent, the boiler is over ten to twelve years old, or spare parts are hard to source. Older, lower-efficiency boilers also cost more to run, so a repeated repair on an ageing unit can be money spent twice.
A few questions help weigh it up:
- How old is the boiler, and is it still under warranty?
- How does the repair cost compare with the value of the unit?
- How many faults has it had in the last year or two?
- Are replacement parts still readily available?
- Would a newer, more efficient model meaningfully cut running costs?
An engineer can give a view on whether a fault is isolated or a sign of wider wear. It is reasonable to ask for the likely cause, the parts involved, and whether further problems are expected before deciding. Getting more than one opinion on a borderline case is sensible, particularly when the figures for repair and replacement are close.