What to do when your boiler breaks down

A boiler always seems to pick the coldest morning of the year to stop. Before you book an engineer, it is worth five minutes of checks, because a good share of breakdowns are not faults inside the boiler at all. Which? lists low pressure and a frozen condensate pipe among the three most common boiler problems, and says two of the three can be handled by a competent DIYer. This guide takes you through the safe checks in the right order, tells you what each fault code is likely to mean, and makes clear where your part stops and a Gas Safe registered engineer's starts.

First: rule out a gas leak or carbon monoxide

Everything else can wait. If you smell gas, HSE says to:

• Call the gas emergency number, 0800 111 999. It is free and open 24 hours a day.
• Open all the doors and windows.
• Turn off the gas at the meter control valve, if you know where it is.

Do not switch lights or appliances on or off, do not use a naked flame, and get everyone out of the house. HSE is clear that it is illegal to use a gas appliance you suspect is unsafe, so leave it switched off until a Gas Safe registered engineer has checked it.

Carbon monoxide is the other danger. It is colourless and has no smell, and its early symptoms are easy to mistake for flu or tiredness. HSE lists headaches, dizziness, breathlessness, nausea, tiredness, chest or stomach pains, and loss of consciousness. If symptoms ease when you go outside, or several people in the house feel the same, treat it as carbon monoxide: switch off the appliance, open the windows, get out, call 0800 111 999 and get medical advice. Signs that a boiler or fire may not be burning properly include a yellow or orange flame where it should be blue, soot or staining around it, a pilot light that keeps going out, and more condensation than usual on the inside of windows. HSE strongly recommends a carbon monoxide alarm that meets British Standard EN 50291.

Check the power, the gas and the controls

If there is no smell of gas and nobody feels unwell, start with the simple things. They catch more breakdowns than you would expect.

• Power. Is the boiler display or light on? Check the fused switch next to the boiler and the trip switches in your consumer unit. A power cut or a tripped circuit will stop the boiler, and it should restart when the power is back.
• Gas. Do your other gas appliances, such as the hob, work? If not, the problem is the supply, not the boiler. On a prepayment meter, check there is credit, including any emergency credit.
• Thermostat and timer. Turn the room thermostat up a few degrees above the room temperature. Check the timer or programmer is set to on and has the right time, especially after a power cut or the clocks changing. Wireless thermostats and smart controls run on batteries, and flat batteries are a common, cheap fix.
• Hot water but no heating, or the other way round. On a combi boiler, if the taps run hot but the radiators stay cold, the problem is more likely the controls, a valve or the pump than the burner, which is useful to tell the engineer.

Check the boiler pressure

Most homes with a combi or system boiler have a sealed heating system with a pressure gauge on or under the boiler. If the pressure drops too low, the boiler stops to protect itself. Worcester Bosch says the gauge should read between 1 and 1.5 bar when the system is cold, usually in the green section, and that below 0.5 bar water has been lost and must be replaced.

Low pressure often follows bleeding radiators, or a long spell with the heating off. Your boiler manual shows how to top it up using the filling loop, usually a braided hose or a built in key under the boiler: open the valves slowly, watch the gauge, close them at about 1 to 1.5 bar, then reset the boiler. Which? notes that if the pressure keeps dropping, you should get an engineer to check for a leak. Worcester Bosch says topping up once or twice a year is normal, so needing to do it every week is a sign something is wrong. Look for drips under the boiler and around radiator valves, and tell the engineer what you found.

Freezing weather? Check the condensate pipe

This is the classic winter breakdown. Modern condensing boilers produce a small amount of acidic water, which drains away through a plastic pipe, usually white or grey and about the width of a thumb, often running outside to a drain. In a cold snap the water can freeze in that pipe and block it. The boiler then shuts down as a failsafe, so that water cannot back up into it.

Baxi says you may hear gurgling from the boiler and see a fault code on the display. The frozen section is usually where the pipe is most exposed: outside, at the end, or at a bend or dip where water collects. To thaw it:

1. Find the frozen section by following the pipe from where it leaves the house.
2. Hold a hot water bottle, a microwavable heat pack or a cloth soaked in hot water against it, or pour on hot, but not boiling, water. Boiling water can crack the pipe.
3. Take care on the ground, because the water you pour can freeze and make it slippery.
4. Once it is clear, reset the boiler following the manual.

Baxi says you should not need an engineer to thaw the pipe, but if the boiler still will not work afterwards, call a Gas Safe registered engineer. To stop it happening again, ask the engineer about waterproof lagging for the outside section, or rerouting a long external run.

Fault codes and resetting the boiler

Most boilers show a fault code when they lock out. The codes are different for every manufacturer, so look yours up in the manual or on the manufacturer's website, where they are usually listed with what the homeowner can safely do. Write the code down before you reset anything: it saves the engineer time, and time is money on a call out.

Resetting once, after you have fixed a simple cause such as low pressure or a frozen pipe, is fine. If the boiler locks out again with the same code, stop resetting. Repeated lockouts usually mean an ignition, flame or fan fault that needs an engineer, and pressing reset again and again can make things worse.

When to call a Gas Safe registered engineer

Call an engineer if the checks above do not fix it, or straight away if you see any of these:

• water leaking from the boiler itself;
• the pressure keeps dropping after you top it up;
• loud banging, whistling or burning smells;
• the same fault code returns after a reset;
• a yellow or orange flame, soot or staining, or a pilot light that keeps going out.

By law, anyone working on the gas side of your boiler must be on the Gas Safe Register, and the back of their ID card shows whether they are qualified for boilers. Our guide on how to check a tradesperson is genuine shows how to check the licence number in two minutes. Never remove the boiler casing yourself. Which? warns against it unless you are a trained heating professional.

When you call, have ready: the boiler make and model (on a label on the front or underneath), the fault code, what you have already checked, whether it is under warranty, and whether anyone in the home is vulnerable to the cold. Ask for the call out charge and the hourly or fixed price before the engineer arrives, as our guide to what a plumber charges per hour explains.

Repair or replace?

Ask for any repair price in writing, and see how much a boiler repair costs for the published price ranges. A single failed part on a boiler that has been looked after is often worth fixing. If the boiler is old, has broken down several times recently, or the engineer says parts are hard to get, ask for a replacement quote as well so you can compare. Unless the boiler is unsafe to use, you have time to get a second opinion.

Planning ahead makes a real difference. In a Which? survey published on 1 October 2025, 33 percent of emergency boiler replacements happened in winter, three quarters of owners of older systems were unprepared for a sudden breakdown, and 27 percent of respondents had heated water on the stove to wash during a breakdown.

How to stop it happening again

• Service it every year. Which? found 31 percent of owners of older systems service less than once a year, and around half of people do not have an annual service at all. A service catches worn parts before they fail, and manufacturers usually require one to keep the warranty valid. See how much a boiler service costs.
• Book before the cold arrives. Engineers fill up with breakdowns from October, so a late summer or early autumn service is easier to get.
• Check the pressure once a month in winter, so you spot a slow leak early.
• Lag the condensate pipe where it runs outside, and the water pipes in the loft and garage too. Our guide on how to stop pipes freezing and what to do if one bursts covers the rest.
• Run the heating briefly now and then in summer, so pumps and valves do not seize.
• Fit a carbon monoxide alarm near the boiler and test it monthly.

Landlords have a separate legal duty to have gas appliances checked every year, covered in our guide to gas safety certificates.

Find a local gas engineer

If your checks have not fixed it, find a local gas engineer and request a quote free. Tell them the boiler make, the fault code and what you have tried, and the business you choose can come back to you with a price and a time. You pay them directly.

Or browse local businesses: gas engineers in Birmingham, gas engineers in Walsall, heating engineers in Coventry, heating engineers in Solihull, plumbers in Wolverhampton or plumbers in Dudley. For more winter and home guides, see all our home advice.