What does your boiler
fault code mean?
Find your boiler brand below and look up the code on your display. Each one shows what it means, what's likely causing it, and whether it's safe to check yourself or needs a Gas Safe registered engineer.
Boiler pressure & radiator air
SAFE TO DO YOURSELFTwo of the most common causes of a fault code are low system pressure and trapped air in the radiators. Both are safe, normal homeowner maintenance — no gas or electrical work involved, just the water side of the heating system.
How do I know if pressure needs topping up?
Check the pressure gauge on the front of the boiler (usually a small dial, sometimes a digital readout). Normal running pressure is 1.0–1.5 bar when the heating is cold. If the needle or reading is below 1 bar, or a low-pressure fault code has come up (F1, H07, E119, L2 — see below), it needs topping up. Above 2.5–3 bar is too high and also needs attention.
How to top up the pressure
Most boilers have a filling loop — a braided silver hose connected to the pipework underneath, with a valve (or two) on it. With the boiler off and cool: slowly open the valve(s) until you hear water filling and watch the gauge rise. Stop as soon as it reaches 1.3–1.5 bar — this is the target, don't overfill. Close the valve(s) fully, then switch the boiler back on. Some boilers need a reset button pressed afterwards to clear the fault.
When it's more than just topping up
Topping up once every few months is normal. If pressure keeps dropping every week or two, that points to a leak somewhere in the system (or a failing component) rather than something to keep topping up — that needs an engineer to find and fix, not repeated top-ups.
How do I know if a radiator needs bleeding?
If a radiator is cold at the top but hot at the bottom, there's trapped air inside stopping it from heating fully — that's the signal to bleed it. Cold all over usually means something else (like a closed valve or pressure issue), not air.
How to bleed a radiator
With the heating on and radiators hot, use a radiator key on the small valve at the top corner of the radiator. Hold a cloth underneath, turn slowly a quarter-turn — you'll hear a hiss of escaping air. Close it the moment steady water (no more air) starts to come out. Check the boiler pressure afterwards, as bleeding radiators often drops it slightly and it may need a small top-up.
Frozen condensate pipe in winter
In freezing weather, the plastic condensate pipe (the small waste pipe running outside from the boiler) can freeze and block, which often triggers a lockout or ignition fault. If it's safe to reach, you can pour warm — not boiling — water along the outside of the pipe to thaw it, then reset the boiler. If you can't reach it safely, it's still frozen after thawing, or the fault returns, that's one for an engineer.
Worcester Bosch
Greenstar / CDi / i RangeCovers the Greenstar i, Si, CDi, 4000, 8000 and Heatslave ranges. Codes beginning with "H" are maintenance notices, not faults — the boiler keeps running.
No flame detected
The boiler tried to ignite but couldn't detect a flame. In freezing weather this is most often a frozen condensate pipe rather than a real fault.
Low water pressure
System pressure has dropped below the safe minimum. Usually fixed by topping up via the filling loop to around 1.5 bar.
Fan fault
The fan or its control circuit isn't operating correctly. Critical for safe flue operation, so needs an engineer.
Sensor fault
A temperature or flow sensor is reading incorrectly or has failed. Try one reset; if it returns, the sensor likely needs replacing.
Pump stuck or running dry
Often linked to low pressure. Check pressure first — if the pump has seized, it needs freeing or replacing by an engineer.
Overheating
Usually caused by poor water circulation. Bleeding radiators and checking pressure can help, but persistent overheating needs inspection.
Vaillant
ecoTEC Plus / ProVaillant fault codes use an "F" prefix and are broadly consistent across the ecoTEC range.
Low water pressure
System pressure is too low for safe operation. Top up via the filling loop; if it keeps dropping, there's likely a leak.
Ignition failure
The boiler couldn't establish a flame at startup — often a blocked burner, faulty ignition electrode, or gas supply issue.
Flame lost during operation
Ignition succeeded but the flame cut out afterwards. Can relate to gas pressure, a blocked flue, or condensate issues.
Pump or pressure sensor fault
The boiler didn't detect an expected pressure change after the pump started — usually a circulation problem, sometimes sludge or limescale related.
Gas valve fault
The gas valve is taking too long to open or close. This is a safety-critical component fault.
Pump or temperature spread fault
Indicates the pump isn't circulating water correctly, often due to debris or a failing pump.
Baxi
600 / 800 / Platinum+ / Duo-tecBaxi codes use an "E" prefix, sometimes displayed as two alternating digits (e.g. E133 flashes as "E1" then "33").
Low water pressure
System pressure below roughly 0.5 bar. Top up using the filling loop to 1.0–1.5 bar; check for leaks if it recurs.
Ignition failure
Boiler failed to ignite — check the gas supply is on and prepayment meter has credit first. Very common cause in winter is a frozen condensate pipe.
Poor primary circulation
Boiler temperature isn't rising as expected after the burner lights, pointing to a circulation issue.
Fan fault
The combustion fan isn't operating correctly — safety-critical, needs inspection.
Flue thermistor fault
The sensor monitoring flue temperature has failed or is reading incorrectly.
PCB fault
A problem with the boiler's printed circuit board or power supply. Often needs board diagnosis or replacement.
Ideal
Logic / Logic Max / VogueIdeal uses F and L prefixes. L5 simply means the boiler has locked out after repeated resets — the underlying cause is usually F1 or L2.
Low water pressure
Pressure has dropped below the safe operating level. Top up to around 1.5 bar via the filling loop.
Ignition lockout
The boiler failed to ignite or lost the flame. Frozen condensate pipe is a common winter cause — otherwise check gas supply and try one reset.
Fan fault
The combustion fan isn't reaching or maintaining the correct speed.
Thermistor fault
A flow or return temperature sensor is faulty, affecting how the boiler regulates heat.
PCB fault
An issue with the main control board. A single power reset can be tried, but recurrence means the board needs inspection.
Repeated lockout
Boiler has reset itself multiple times in a short period — a safety feature, not a new fault. The root cause (usually F1 or L2) needs diagnosing.
Glow-worm
Flexicom / Betacom / EasicomGlow-worm is part of the Vaillant Group and shares similar fault logic. Betacom models use flashing LED colours instead of a digital code.
Low water pressure
Top up via the filling loop to 1–1.5 bar. Recent, sudden drops usually mean a leak somewhere in the system.
Ignition fault / frozen condensate
Boiler failed to detect a flame at startup. In cold weather this is very often a frozen external condensate pipe.
Fan fault
Fan can't reach the speed needed for safe ignition — a mechanical or wiring issue.
Flue or fan sensor fault
Wiring or sensor issue affecting safe flue operation.
Main board or gas valve fault
A fault within the control board or gas valve circuit — always needs professional diagnosis.
Overheating
Boiler temperature has exceeded safe limits, usually linked to circulation problems.
Fault code not clearing?
If a reset doesn't hold, or the code points to ignition, gas, sensor or PCB issues, it's time for a Gas Safe registered engineer. Same-day visits across North & North West London, plus Watford, Slough & Uxbridge.