Boiler fault codes
Boiler Fault Codes by Brand
A fault code on a boiler tells you which safety check it failed. Find the brand on the front panel or data badge, then the code below. Low pressure codes, and ignition lockouts caused by a frozen condensate pipe, you can often clear yourself. Overheat, fan, flue and sensor codes need an engineer.
Read the code before you press reset
Take a photo of the display first. A reset wipes what is on screen, and the engineer will want the exact code and whether it came back straight away, after a few minutes, or only when you ran a hot tap.
Brands write codes differently. Vaillant shows an F and a number, such as F28 for a failed ignition. Worcester Bosch often pairs a two-letter display code with a three-digit cause code, so EA 227 is a no-flame lockout. Baxi uses E numbers, and on many models E119 means low system pressure. Ideal uses F codes, where F1 is low pressure and F2 is flame loss.
Codes you can often clear yourself
Low pressure codes clear once the system is topped up to around 1 to 1.5 bar on the gauge using the filling loop, with both loop valves closed again afterwards, then reset once. If the code returns within days, there is a leak or a failing expansion vessel and topping up is only buying time.
Ignition codes in winter are often a frozen condensate pipe outside. Thawing it with warm water and one reset is safe. Also check the gas is on at the hob and, on a prepayment meter, that there is credit.
Codes that mean leave it alone
Overheat, flue, fan and repeated flame-loss codes point to parts inside the sealed case: the fan, gas valve, electrodes, sensors or the printed circuit board. Behind the case sit the sealed combustion chamber and the gas components, so opening it is a job for a Gas Safe registered engineer, never a householder. Gas work is carried out by Gas Safe registered engineers.
If the same ignition code keeps returning after one reset, stop. Our brand pages, starting with Vaillant fault codes and Worcester Bosch fault codes, list which part is usually behind each code and what it tends to cost. A visit is the £60 call-out at any hour, and the price of any repair is agreed with you on site before work starts.
A code with a smell of gas is a gas emergency, not a fault code. Open windows, turn the gas off at the meter if safe, avoid switches, then leave and call 0800 111 999.
Vaillant
- All Vaillant codes
- Vaillant F20The boiler's overheat protection has tripped: the water temperature at the flow or return sensor went over the safety limit, so the boiler closed the gas valve and shut itself off.
- Vaillant F22The boiler has switched itself off because there is not enough water, or not enough water pressure, in the heating circuit. Vaillant calls it a safety switch-off for low water pressure.
- Vaillant F23The gap between the flow temperature (water leaving the boiler) and the return temperature (water coming back) is too large, which tells the boiler that water is not circulating properly. It shuts down to protect the heat exchanger.
- Vaillant F24The temperature inside the boiler rose faster than it should after the burner lit, which means water is not flowing through the heat exchanger properly. The boiler shuts down before it can overheat.
- Vaillant F25A safety shut-off: the boiler has detected, or thinks it has detected, flue gas temperature that is too high, and has switched itself off.
- Vaillant F28The boiler tried to light on start-up, never detected a flame, and locked itself out. Vaillant describes it as the flame signal not being detected during the ignition phase.
- Vaillant F29The boiler was running, lost its flame, and the attempt to relight failed. Vaillant describes it as unsuccessful ignition following a flame loss during operation.
- Vaillant F32A fan fault: the boiler cannot confirm its fan is running at the right speed, so it switches off rather than light the burner.
- Vaillant F61F.61 means the boiler has found a fault in the circuit that drives its gas valve, so it has shut the gas off and locked out. The cause is in the valve, its wiring or the control board, all inside the casing.
- Vaillant F62F.62 means that when the boiler told the gas valve to close, the flame took too long to go out. The boiler reads that as a gas valve that may not be shutting off cleanly, and locks out.
- Vaillant F75F.75 means the boiler switched its pump on and the water pressure sensor did not register the small pressure rise it expects. Low pressure, trapped air, a failing pump or a faulty pressure sensor are the usual causes.
- Vaillant F83F.83 means that when the burner lit, the flow or return temperature sensor registered little or no rise in temperature. The boiler reads that as water not moving through the heat exchanger properly, or a sensor reading wrongly, and shuts down.
Worcester Bosch
- All Worcester Bosch codes
- Worcester Bosch 1101Fault 1101 is widely reported as the boiler losing contact with its room thermostat or programmer, so it stops getting heating demands. Worcester does not list 1101 in the control manuals we checked, so treat it as a controls fault and note every code on screen.
- Worcester Bosch A21On Worcester Comfort RF wireless controls, A21 means the room thermostat has lost its radio link with the receiver, so the boiler is no longer getting heating demands. On a wired Sense I control, A21 with sub-code 1001 means the control has been set up wrongly instead.
- Worcester Bosch C6The boiler measured its fan running at the wrong speed and locked out, because it cannot burn gas safely without the right amount of air. On Greenstar models, 215 after C6 means too fast and 216 means too slow.
- Worcester Bosch E9An overheat safety device inside the boiler has tripped and the boiler has locked out to protect itself. The three-digit cause code shown with E9 says which sensor tripped and why.
- Worcester Bosch EA 227The boiler sparked to light the burner but never sensed a flame, so it shut down and locked out. Worcester lists 227 as "no flame signal after ignition"; it stays locked until someone presses reset.
Baxi
- All Baxi codes
- Baxi E1On most Baxi boilers with a two-digit display, E1 is not a fault on its own: it is the first half of a three-digit code, shown as E1 then two more digits. The digits that follow tell you the real fault.
- Baxi E119E119 means the water pressure in the heating circuit has fallen below the minimum the boiler will run on, about 0.5 bar. The boiler shuts down until the pressure is restored.
Ideal
- All Ideal codes
- Ideal F1The boiler has measured the water pressure in the heating system as too low to run safely, so it has stopped. Ideal says the pressure needs topping up to between 1 and 1.5 bar.
- Ideal F2F2 is flame loss. The boiler lit, or tried to, but the flame sensor stopped seeing a flame, so the gas valve closed and the boiler stopped.
- Ideal F3F3 is a fan fault. The control board has not seen the combustion fan run as it should, so the boiler will not light, because it cannot prove the flue gases will be cleared.
- Ideal F4On an Ideal boiler, F4 (L4 on some control boards) is a flow thermistor fault: the board is getting a reading from the flow temperature sensor that makes no sense, so it stops the boiler.
Glow-worm
- All Glow-worm codes
- Glow-worm F1On older Glow-worm boilers F1 is an ignition fault: the boiler tried to light, never detected a flame, and locked out. On the newer Energy and Ultracom2 ranges the similar-looking F.01 or F01 means something else, a return temperature sensor fault.
- Glow-worm F9On older Glow-worm boilers F9 is a water pressure fault. Most often the system simply needs repressurising, while Glow-worm’s engineer tables list a faulty water pressure sensor, a blocked flow filter or a pump fault.
Potterton
Viessmann
- All Viessmann codes
- Viessmann F2The safety temperature limiter has responded: water inside the boiler got hotter than it should, so the burner locked out. Viessmann’s Vitodens 100-W service manual lists F2 as “the temperature limiter has responded”.
- Viessmann F4The boiler tried to fire but no flame signal was detected, so it locked out. Viessmann’s Vitodens 100-W service manual lists the cause of F4 as “no flame signal”.
Vokera
Questions people ask
Can I clear a boiler fault code by turning it off and on?
Often not. Many boilers are designed to hold an ignition or overheat lockout through a power cut, so only the reset button clears it. Where switching off at the fused spur does clear the screen, it clears the display, not the cause. Write the code down first and try it once.
My brand is not listed. What now?
The data badge, usually behind the front flap or on the underside of the boiler, gives the make and model. Search the code with the model name, or tell the engineer the exact code when you book.