Boiler Keeps Cutting Out? Causes and Fixes

A boiler that fires up and then shuts down is doing it deliberately. A safety device has detected something it does not like. The job is working out which one.

Some of the causes you can check and fix yourself in a few minutes. Others need a Gas Safe engineer. This guide tells you which is which.

First, check for an error code

Most modern boilers display a code when they cut out. Note it down, because it narrows the diagnosis enormously.

Our boiler error codes guide explains what the common codes mean and what to do about each one. If there is no code, or the display is blank, work through the causes below.

What you can check yourself

Low pressure

The most common cause by a distance. Below 1 bar most boilers shut down as a safety precaution.

The gauge should read between 1 and 1.5 bar, usually in a green zone. If it is low, top it up using the filling loop. If it keeps dropping, you have a leak and topping up is not the answer.

Topping up is a safe and easy job to do yourself. Our guide to a boiler losing pressure walks through it step by step, including how to find the leak.

Frozen condensate pipe

In cold weather the external condensate pipe, which carries waste water from the boiler to a drain outside, freezes and blocks. That shuts the boiler down.

The signs are obvious: it is freezing outside, the boiler worked fine until the cold arrived, and there is a white plastic pipe running outside. Pour warm, not boiling, water over the frozen section and reset. In most cases that is all it takes. Insulating the pipe prevents it recurring.

Thermostat problems

Sometimes the boiler is fine and the thermostat is the issue. Check it is set higher than the current room temperature, that wireless thermostat batteries are not flat, that it is not sitting in direct sunlight or next to a radiator, and that the programmer has not been knocked.

Causes that need an engineer

If the checks above do not solve it, the cause is probably one of these. They all need proper diagnosis, and anything involving the gas side of the boiler needs a Gas Safe registered engineer.

Ignition failure. The boiler tries to light and fails. You may hear clicking, or see an ignition error on the display. Usually faulty ignition leads, dirty electrodes, a gas valve problem or a blocked burner. Not a DIY job.

Fan fault. The fan provides airflow for safe combustion. If it is not running correctly the boiler shuts down almost immediately. The typical sign is a boiler that starts then stops quickly, with no sound of the fan running. The fan may need cleaning, repair or replacement.

Pump failure. The pump circulates water. If it fails, the boiler overheats and cuts out. Radiators stay cold and you may hear gurgling. A stuck pump can sometimes be freed. Otherwise it needs replacing.

Overheating. When the boiler gets too hot, safety devices shut it down before the heat causes damage. Caused by pump failure, sludge restricting flow, a blocked heat exchanger or limescale. Often needs a powerflush rather than a part, though pump repair or descaling can be the answer.

PCB fault. The circuit board is the boiler’s brain. When it fails you get random shutdowns, odd error codes and inconsistent behaviour. It needs repair or replacement, and it is usually the most expensive fix.

Blocked heat exchanger. The heat exchanger transfers heat from the burning gas into the water, so a blockage makes the boiler run hot. Kettling noises alongside frequent cut-outs point here. Cleaning or a powerflush may resolve it, or it may be damaged and need replacing.

Stop after three resets

If you have reset it three times and it keeps cutting out, stop.

The boiler is shutting down because something is wrong. Forcing it to restart repeatedly can damage components further, may void your warranty, and in rare cases creates a genuine safety issue. At that point it is time to call an engineer.

What the timing tells you

When it cuts out	Likely cause
Immediately on startup	Ignition failure or gas supply
After a few minutes	Overheating, pump failure, blocked system
When hot water is used	Diverter valve, flow sensor, low pressure
Randomly	PCB fault, loose connection, thermostat
Only in cold weather	Frozen condensate pipe
On heating but not hot water	Circulation problem, sludge, pump

How repair costs compare

The cost depends on your boiler and on which part has failed, so any figure given before diagnosis is a guess. In rough order, from least to most expensive:

1. A pressure top-up or basic checks
2. A new ignition electrode
3. A new fan or pump
4. A new PCB or heat exchanger, which sit at the top end

We diagnose before quoting, so you know the price of a boiler repair before any work starts.

Preventing it

• Annual service. Most of the faults above are found and dealt with before they strand you. An engineer checks every component and catches problems early. A boiler service costs from £105.
• Check pressure monthly. A slow drop noticed early is a small job. Top up if it falls below 1 bar. Frequent pressure loss means a leak.
• Fit a magnetic filter. Catches the debris that causes blockages and overheating, and extends the life of the boiler.
• Insulate the condensate pipe. If it runs outside, this removes an entire category of winter breakdown. It can be done during a service.