BoilerBuddy · symptom guide
Boiler keeps turning off — the five things that drive it.
A boiler that fires for a few minutes, runs, then goes off with a flashing code, a quiet cut-off, or a click — and may keep doing that every time you reset — is the system telling you what's stopping it. The five causes, ranked most-likely-first, are low system pressure tripping the pressure switch, a thermostat or programmer sending a "boiler off" signal, an overheat thermostat tripping on scale or low flow, a frozen condensate pipe pushing the pressure switch out, or a flame-loss / air-pressure interlock stopping the burner. The cards below rank them most-likely-first; the safe checks that follow let you walk through the five in order.
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Why this happens
Five causes, ranked by how often we see them on a UK combi.
A boiler that fires then goes off is the pressure side, the controls side, the heat side, or the flame side telling you no. The five common culprits, most-likely-first, are system pressure sitting below 1.0 bar tripping the pressure gauge lock-out, a stuck thermostat or programmer calling a "boiler off" before the run finishes, scale or low flow letting an overheat thermostat trip, a frozen condensate pipe on an outside-wall run, and a flame-sensing flame-sensing electrode or air-pressure switch cutting the burner. Each has a different sign; the cards describe each.
Low system pressure — the pressure gauge reading isn't right
A combi needs the system pressure sitting in the green band on the pressure gauge before it will stay running. If the pressure has dropped under 1.0 bar when cold, or the pressure relief valve has been dripping overnight, the pressure switch trips within a few seconds of firing and the boiler goes off with a low-pressure code. Top up via the filling loop until the needle sits between 1.0 and 1.5 bar when cold; if it drops again within a day or two, the cause is on the system side and the related pressure-loss guide covers that framing.
Safe at home · read the gauge and top up if low
Run this in the triage →Thermostat or programmer sending the boiler off mid-run
A room thermostat with a worn internal switch can throw a "boiler off" signal moments after a firing starts, the boiler returns to idle with no code, and you read it as the boiler going off. The same happens on a system or regular boiler when a cylinder thermostat sticks or schedules a hot-water slot that doesn't reach the cylinder. Turn the thermostat up to 25 °C and watch whether the boiler re-fires — if it does, you've found it. That one's a five-minute fix, not engineer territory.
Safe at home · turn it up and retest
Run this in the triage →Overheat thermostat tripping on scale or low flow
An overheat stat on the heat exchanger trips when scale build-up or low flow lets the water get too hot, then resets once it cools, only to trip again the next time you run the boiler. The symptom is the same cycle — fire, go off, fire — with a kettle-like rumble once or twice before the cut-out. That's the kettling page's framing in full, with the same 'do not descale it yourself' line. Engineer territory; the fix is usually a descale, a power flush, or a heat-exchanger replacement depending on the age of the boiler.
Engineer territory · descale, flush or replace
Run this in the triage →Frozen condensate pipe — outside-wall boiler in winter
The boiler fires, water leaves the heat exchanger, the condensate freezes where the pipe runs outside, the backed-up condensate can't reset the pressure switch, and the boiler locks out within a minute. Warm the pipe from the outside with warm (NEVER boiling) water, confirm the boiler then runs, and insulate the pipe before the next frost. Pressure-side symptoms — the pressure dropping, the pressure gauge dropping overnight — are the related pressure-loss page's domain.
Safe at home · warm the outside pipe then insulate
Run this in the triage →Flame-loss or air-pressure interlock — the burner won't stay lit
The boiler ignites, runs for one to five minutes, then goes off with a flashing fault code. This is the family of faults that includes the flame-sensing electrode, the gas valve, the air-pressure switch, and the PCB — but the difference here is that the boiler DOES start, it just won't stay lit. Note the code exactly, look it up under your brand, and book a registered engineer if the same code comes back after a reset.
Engineer territory · if it repeats after a reset
Run this in the triage →Safe at home
Five checks to do before you call anyone.
These five checks walk through the five causes above in order. None of them requires opening the boiler or touching the gas side. Start by reading the pressure gauge and the thermostat position — the two most common causes clear without an engineer — and work down the list before reaching for the reset button.
Read the pressure gauge and top up if low
Stand in front of the boiler and read the small round pressure gauge. Under 1.0 bar when cold is low — use the filling loop to bring the needle into the green band (1.0–1.5 bar). If the pressure drops again within a day, the cause is on the system side and is the related pressure-loss guide rather than this one.
If the boiler keeps turning off after the five, stop here and book an engineer.
Turn the thermostat up and watch the programmer
Turn the room thermostat up to 25 °C, walk to the programmer, and confirm it's set to "always" for both heating and hot water. A programmer toggled to "heating only" or a smart schedule that has reset is the most common cause of a boiler that seems to go off mid-run with no fault code.
If the boiler keeps turning off after the five, stop here and book an engineer.
Note exactly what the boiler is showing
If the boiler has gone off, note everything on the display — the code text or flashing-LED pattern, whether the burner lights at all, whether the pump runs, and how long the boiler runs before it goes off. A photo of the display is the single most useful thing to hand an engineer. Don't keep resetting past a single attempt if the same symptom comes straight back.
If the boiler keeps turning off after the five, stop here and book an engineer.
Go outside and check the white condensate pipe
Modern condensing boilers have a small white plastic pipe — usually exiting through an outside wall. In winter, walk outside and look at it. If there's ice on the pipe or a frozen gurgle at the outlet, that's likely the cause. Pour warm (NOT boiling) water over the frozen section, confirm the boiler runs, and insulate the pipe before the next frost. If it's not cold outside or there's no visible ice, this isn't the cause.
If the boiler keeps turning off after the five, stop here and book an engineer.
Know when to stop
Stop if you notice
- A smell of gas, or a yellow, orange or sooty flame.
- A carbon monoxide alarm is sounding, or someone has possible CO symptoms.
- Smoke, scorching, visible damage, overheating or a leak around the boiler.
- The boiler keeps locking out or will not stay lit.
Try this first
Read the gauge and the thermostat, then reset once.
The two most common "going off" causes clear without an engineer visit. Read the pressure gauge and top up via the filling loop if it sits under 1.0 bar. Confirm the thermostat is set to 25 °C and the programmer is on "always". Reset once and wait. If the boiler then fires and stays running, the cause was on the pressure side or the controls side. If it goes off again on the second run, the fault is past the safe-at-home line.
8 steps. Yes / no at each. Stops the moment a step says it should.
Where the safe-at-home line stops
If any of these apply, stop and call a Gas Safe engineer.
These are the lines that mean the fault is past the safe-at-home boundary. Any of them — leave the boiler off, leave the heating off, and book a registered engineer today, not next week:
- The boiler goes off within a few minutes of every reset — a single reset is the safe-at-home line; if the fault keeps coming back after the second run, the cause is a failing sensor, a flame-sensing issue, an air-pressure interlock, or a frozen condensate the reset can't clear, and that's registered-engineer territory.
- You can smell gas, your CO alarm is sounding, or the display is blank on a boiler that was firing yesterday — stop, leave the property, and read the emergency safety guide before doing anything else.
Next step
Run the full triage in about a minute.
Tell BoilerBuddy what your boiler is doing. We will return a ranked list of likely causes, the safe checks to do first, and the exact line that means 'call a Gas Safe registered engineer'.
FAQ
The five questions this symptom raises.
Fault-code guides for exact boiler models
Need a qualified engineer?
BoilerBuddy can pass what you've worked out to a local Gas Safe engineer — symptoms, checks, and the bits that point to the likely cause.
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Engineer handover
Send this sheet with you to the visit.
Print or email it to your engineer so they arrive already knowing what your boiler is doing and what you've already tried.