Boiler Not Working? What to Check Before You Call

Updated July 2026 · Emergency Plumber Chelsea

First, be clear about which symptom you have, because it narrows the search. A completely blank display with no lights means a power problem. Lights on but no heating and no hot water points to pressure, gas, a lockout or a control fault. Hot water but no heating usually means the timer, thermostat, a zone valve or the diverter valve. Heating but no hot water on a combi points to the diverter valve or the hot water sensor. If your symptom is one of the last two, our guides to no heating but hot water and no hot water go into more detail than this one.

Check the power next. Most boilers are wired to a fused spur switch on the wall nearby, often with a small neon light. Make sure it is switched on and the light is lit. Look at the consumer unit for a tripped switch, and if a trip has gone, reset it once. If it trips again straight away, leave it off and call an engineer, because something on that circuit has a fault. After a power cut, some boilers need a minute or two to run through their start-up before they show any sign of life.

Then check the controls, because a boiler that is not being asked for heat looks exactly like a broken one. Look at the programmer or timer: after a power cut or a clock change it may have reset to the wrong time or switched to off. Turn the room thermostat up a few degrees above the room temperature and listen for a click. Wireless and smart thermostats run on batteries or a Wi-Fi link, and flat batteries or a lost connection are a very common reason for a boiler that will not fire up. If the boiler starts as soon as you put the timer to constant or override, the fault is in the controls, not the boiler.

Check the pressure gauge on the front of the boiler. On a sealed system, combi or system boiler, the needle should normally sit between about 1 and 1.5 bar when the system is cold. If it has dropped below 1, or towards zero, most boilers will refuse to fire and show a low-pressure code. You can usually top it up using the filling loop underneath. If it drops again within days, there is a leak or a faulty pressure relief valve, and our guide to a boiler losing pressure explains how to top up and what to look for next. If the needle is well above 2.5 bar instead, see our guide to boiler pressure too high.

Check the gas. Try a gas hob if you have one. If that does not light either, the problem is the supply, not the boiler. On a prepayment meter, check there is credit, because the supply cuts off when it runs out and the boiler cannot fire. Check that the lever on the emergency control valve by the meter is in line with the pipe, which means on. If the whole street is without gas, the network operator will know. Do not touch the gas valve if you can smell gas; instead, follow our guide on what to do if you smell gas in your house.

In a cold snap, check the condensate pipe. Modern condensing boilers drain a small amount of acidic water through a plastic pipe, and where that pipe runs outside or through a cold space it can freeze. The boiler then locks out, and you may hear a gurgling noise before it stops. Pouring warm, not boiling, water along the outside section usually thaws it, and the boiler can then be reset. Our guide to a frozen condensate pipe shows how to find the blockage and thaw it safely.

If the boiler shows a fault code or a lockout light, reset it once. Every manufacturer puts the reset in a slightly different place, and our guide on how to reset a boiler explains where to find it and what to check first. A boiler that fires after one reset and then runs normally has probably had a one-off ignition failure. A boiler that locks out again and again has a real fault, such as a failing ignition electrode, a flame sensing problem, a gas valve fault, a blocked flue or fan problem, or a failing pump. Pressing reset repeatedly does not fix those and can make some of them worse. Note the code before you call, and check it against our guides to Worcester, Ideal or Baxi fault codes. If the boiler does not lock out but fires and cuts out every few minutes, see our guide to a boiler that keeps turning on and off.

Older boilers with a permanent pilot light are becoming rare in Chelsea, but they are still found in some flats and basements. If the pilot has gone out, the manufacturer's relighting instructions are usually printed inside the front panel. If the pilot will not stay lit, the thermocouple or the gas supply to the pilot is usually at fault, and that is a job for a Gas Safe engineer rather than something to keep trying.

Know when to stop. Do not take the casing off the boiler; everything behind it is Gas Safe work. Switch the boiler off and call for help straight away if there is a gas smell, if a carbon monoxide alarm sounds, if you see black or sooty marks around the boiler or the flue, if the flame looks lazy and yellow instead of crisp and blue on an older boiler, or if water is leaking from inside the case. Our guide to the signs of carbon monoxide from a boiler explains what to watch for, and our guide to what to do if your boiler leaks covers leaks.

While you wait, keep warm safely. Use an oil-filled or fan heater plugged directly into a wall socket rather than an extension lead. If you have an immersion heater on a hot water cylinder, it can give you hot water while the boiler is off. Never use a gas cooker or an outdoor heater to heat a room.

If the boiler still will not work after these checks, keeps locking out, or you have found a leak or a pressure problem you cannot solve, call Emergency Plumber Chelsea on 020 3002 6667. Our boiler repair service uses Gas Safe registered engineers who diagnose ignition, gas valve, pump, fan and control faults across SW3, SW10, Knightsbridge, Sloane Square, South Kensington and Fulham. There is no call-out fee on jobs over 3 hours.

Quick tips

• Blank display: check the fused spur switch next to the boiler and the consumer unit first.
• Turn the room thermostat up and put the timer on constant to rule out the controls.
• Pressure below about 1 bar stops most combi and system boilers from firing.
• If the gas hob will not light either, the problem is the gas supply or meter, not the boiler.
• Reset once. A boiler that keeps locking out needs an engineer, not more resets.

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