Before you pick up the phone, work through this checklist — several common boiler faults have simple fixes that take two minutes and cost nothing.
A boiler that stops working is stressful, especially in the colder months. But not every boiler fault requires a Gas Safe engineer and a call-out charge. A surprising number of boiler problems have straightforward causes that any homeowner can diagnose and sometimes fix themselves — no tools, no gas knowledge required.
This guide walks you through the checks to make before calling. If you work through all of these and the boiler is still not working, that's when to call a registered gas engineer in Croydon or your local area.
Modern boilers display fault codes on a small screen or via flashing lights. These codes point directly to the problem and are listed in your boiler manual. Common codes across major brands relate to low pressure, ignition failure, sensor faults, or flue blockages. Look up the code in the manual before anything else — it may tell you exactly what has gone wrong and whether it is something you can reset yourself.
This is one of the most common causes of a boiler lockout. The pressure gauge is usually a dial or a digital reading on the front of the boiler. It should read between 1 and 1.5 bar when the heating is cold. If it reads below 0.5 bar, the boiler has lost pressure and will not fire. You can re-pressurise most combi boilers yourself using the filling loop — a small silver or grey valve (sometimes a flexible hose) usually located underneath the boiler. Your boiler manual will show you exactly where it is and how to use it. Add pressure slowly and stop at 1.2–1.5 bar.
Try a gas hob or gas fire. If those are also not working, the problem is with your gas supply rather than the boiler. Check whether your gas meter shows a fault, or contact Cadent Gas (0800 111 999) if you suspect a supply interruption. If the gas hob works fine, your supply is intact and the fault is within the boiler itself.
This sounds obvious, but it catches people out regularly. Check that the room thermostat is set above the current room temperature, that the programmer or timer is set to "heating on" for the current time period, and that no one has accidentally switched the system to "hot water only" mode. Smart thermostats such as Nest, Hive, or Tado can also lose their connection and need reconnecting to the app.
All modern condensing boilers (fitted since around 2005) have a white plastic condensate pipe that carries waste water outside the property. In cold weather, this pipe can freeze solid, causing the boiler to lock out. The pipe usually exits through an outside wall or into a drain. If there has been a hard frost and your boiler stopped working overnight, this is the most likely culprit. You can thaw it yourself by pouring warm (not boiling) water over the frozen section, then resetting the boiler. Boiling water can crack the plastic pipe.
Boilers require electricity to operate even though they burn gas. Check your consumer unit (fuse box) to see if any circuit breakers have tripped. If the boiler's MCB (the switch labelled "boiler" or "heating") has tripped, reset it by switching it fully to the off position and then back on. If it trips again immediately, do not keep resetting it — call an engineer, as there is likely a fault causing the trip.
Once you have checked all of the above, try pressing the boiler reset button. On most modern boilers this is a clearly labelled button on the front panel, often marked with a flame symbol and an arrow. Hold it for 3–5 seconds. The boiler should attempt to fire. If it fires and then locks out again within a few minutes, there is an underlying fault that needs an engineer. If it fires and runs normally, you are done.
Having worked through the checks above, call a registered gas engineer if:
The best way to avoid a mid-winter boiler breakdown is an annual service — ideally in late summer or early autumn, before the heating season begins. A service takes around 45–60 minutes, involves a thorough check of all internal components, and gives an engineer the chance to spot small faults before they cause a full lockout on the coldest night of the year.
For landlords in Croydon and South London, an annual boiler service is also usually required to maintain the boiler warranty and is closely linked to the landlord gas safety certificate (CP12) that the law requires you to hold and renew every 12 months.
Nasser at Nas Plumbing services all major boiler brands across Croydon, Sutton, Bromley, Mitcham, Streatham, Thornton Heath, Purley, and the surrounding South London area. If your boiler isn't responding to any of the checks above, give us a call.
Call Nas Plumbing in Croydon — Gas Safe registered, fully insured, and covering all of South London. We'll diagnose the fault and give you a clear, honest price before any work starts.
Call 07474 686888 →