Answer six quick questions to find the likely cause, and what's safe to check yourself before you book an engineer.
Smell gas, or has your carbon monoxide alarm gone off? Stop. Turn off the gas at the meter if you can, open windows, leave the property and call the National Gas Emergency Service on 0800 111 999.
It's on the front of the boiler.
The dial or number on the front or underneath the boiler. Read it with the heating off.
What the most common codes mean, and what you can safely check. Worcester Bosch boilers show a letter code with a three-digit number under it (for example EA and 227). Meanings can vary between models, so your boiler's manual has the final say.
F.0
Vaillant
The flow temperature sensor has been interrupted. An electrical fault or a faulty sensor.
Needs an engineer.
F.1
Vaillant
The return temperature sensor has been interrupted. An electrical fault or a faulty sensor.
Needs an engineer.
F.10
Vaillant
The flow temperature sensor has short-circuited. An electrical fault or a faulty sensor.
Needs an engineer.
F.11
Vaillant
The return temperature sensor has short-circuited. An electrical fault or a faulty sensor.
Needs an engineer.
F.12
Vaillant
The hot water temperature sensor has short-circuited. An electrical fault or a faulty sensor.
Needs an engineer.
F.13
Vaillant
The cylinder temperature sensor has short-circuited. An electrical fault or a faulty sensor.
Needs an engineer.
F.20
Vaillant
The safety thermal cut-out has switched the boiler off because it got too hot. Often low pressure or poor circulation.
You can check: Check the pressure gauge, look for leaks, make sure radiator valves are open and bleed any radiators that are cold at the top.
F.22
Vaillant
Low water pressure in the heating system.
You can check: If the gauge reads below 1 bar, top it up to between 1 and 1.5 bar with the filling loop. If it keeps dropping, there's a leak or a faulty expansion vessel.
F.23
Vaillant
The temperature difference between flow and return is too great. Usually air in the boiler or poor circulation.
You can check: Check the pressure gauge and bleed any radiators that are cold at the top.
F.24
Vaillant
The temperature is rising too quickly. Usually air, low pressure or poor circulation.
You can check: Check the pressure gauge and bleed any radiators that are cold at the top.
F.25
Vaillant
The flue gas temperature is too high.
Needs an engineer.
F.27
Vaillant
A flame signal was detected while the burner was off. A safety lockout, often the gas valve or circuit board.
Needs an engineer.
F.28
Vaillant
The boiler tried to light but no flame was detected. Common causes are no gas, an empty prepayment meter or a frozen condensate pipe.
You can check: Check your other gas appliances work and your prepayment meter has credit. In freezing weather, thaw the condensate pipe with warm (not boiling) water, then reset the boiler once.
F.29
Vaillant
The flame went out and the boiler couldn't relight. Common causes are an interrupted gas supply or a frozen condensate pipe.
You can check: Check your other gas appliances work and your prepayment meter has credit. In freezing weather, thaw the condensate pipe with warm (not boiling) water, then reset the boiler once.
F.32
Vaillant
Fan fault.
Needs an engineer.
F.35
Vaillant
The air or flue pipe is blocked.
You can check: If it's safe to reach at ground level, check the flue outside isn't blocked and the condensate pipe isn't frozen.
F.52
Vaillant
Mass flow sensor fault.
Needs an engineer.
F.53
Vaillant
Combustion regulation fault, often low gas pressure.
You can check: Check your gas supply and prepayment credit, and that the condensate pipe isn't frozen.
F.54
Vaillant
Gas supply fault, usually shown with F.28 or F.29.
You can check: Check your gas supply and prepayment credit, and that the condensate pipe isn't frozen.
F.57
Vaillant
The boiler isn't burning correctly in comfort mode.
You can check: Check your gas supply and prepayment credit, and that the condensate pipe isn't frozen.
F.62
Vaillant
The flame took too long to go out after switch-off. Usually the gas valve, circuit board or ignition electrode.
Needs an engineer.
F.70
Vaillant
The boiler's device number is missing or wrong, usually after a part has been replaced.
Needs an engineer.
F.72
Vaillant
The flow and return sensors are giving readings that don't make sense.
Needs an engineer.
F.75
Vaillant
No pressure change was detected when the pump started. Low pressure, air in the system, or a faulty pump or pressure sensor.
You can check: If the gauge reads below 1 bar, top it up to between 1 and 1.5 bar with the filling loop and bleed the radiators.
F.83
Vaillant
Little or no temperature rise when the burner starts. Usually low pressure or poor circulation.
You can check: Check the pressure gauge and top it up if it's below 1 bar.
F.84
Vaillant
The flow and return temperature readings don't make sense, often crossed pipework or misplaced sensors.
Needs an engineer.
F.88
Vaillant
The electrical connection to the gas valve has been interrupted.
Needs an engineer.
EA 227
Worcester Bosch
No flame detected. Usually no gas or low gas pressure, a blocked or frozen condensate pipe, or a blocked flue.
You can check: Check your other gas appliances work and your prepayment meter has credit. In freezing weather, thaw the condensate pipe with warm (not boiling) water, then hold reset for five seconds and wait two to three minutes.
D5
Worcester Bosch
Frozen condensate pipe. The boiler often makes a gurgling noise too.
You can check: Thaw the condensate pipe where it's most exposed with warm (not boiling) water or a hot water bottle, only if it's at ground level, then reset the boiler.
1017
Worcester Bosch
System pressure too low (Greenstar 4000 and 8000).
You can check: Top the pressure up to between 1 and 1.5 bar with the filling loop or key. If it keeps dropping, there's a leak or a faulty expansion vessel.
A1
Worcester Bosch
The pump has run dry, a sign of low water in the system.
You can check: Check the pressure gauge and top it up to between 1 and 1.5 bar if it's low, then reset the boiler.
E9
Worcester Bosch
The safety temperature limiter has tripped because the boiler got too hot.
You can check: Check the pressure is between 1 and 2 bar and top it up if it's low, then hold reset for three seconds. If E9 comes back, it needs an engineer.
C6
Worcester Bosch
Fan speed too low, a fan fault.
Needs an engineer.
SE
Worcester Bosch
Not a fault. The boiler has run for about a year and is due a service.
You can check: Book an annual service.
Sources: Vaillant's fault code list and Worcester Bosch's Greenstar user instructions and technical bulletins.
With the heating off and the boiler cool, find the filling loop. It's usually a silver braided hose or a pair of taps under the boiler, or a key or lever on some Worcester Bosch models. Open it slowly and watch the pressure gauge. Close it when the gauge reads between 1 and 1.5 bar, then reset the boiler if it has locked out. Your boiler's manual shows exactly where the filling loop is. If you have to top it up more than once every few months, there's a leak or a faulty expansion vessel somewhere.
The condensate pipe is the plastic pipe, usually white or grey, that runs from the boiler to a drain outside. In freezing weather it can ice up, often at the open end, a bend or a dip, and the boiler shuts down and may gurgle. Pour warm (not boiling) water along the frozen section, or hold a hot water bottle or warm cloth against it. Boiling water can crack the pipe. Only do this if the pipe is at ground level, and watch for the water freezing into a slip hazard. Once it's thawed, reset the boiler.
Once, after you've fixed the likely cause, such as topping up the pressure or thawing the condensate pipe. If the same fault comes back, don't keep resetting it, especially for an ignition or flame fault. The boiler is locking out for a reason and needs an engineer to look at it.
On a combi boiler, the most common cause is a sticking diverter valve, which switches the boiler between heating and hot water. A faulty hot water flow sensor is another. Both are engineer jobs. On a system or regular boiler with a hot water cylinder, first check the programmer is set to give hot water and the cylinder thermostat is set to around 60°C. If those are fine, the motorised valve may have failed.
Pressure should stay between about 1 and 2 bar. If you keep having to top it up, water is escaping somewhere: a leaking radiator valve, a pipe under the floor, or the pressure relief valve dripping through its pipe outside. A faulty expansion vessel can also cause pressure to swing up when the heating is on and drop when it cools. Bleeding radiators lowers the pressure too, so check it after you bleed them.
You can safely check the pressure and top it up, thaw a frozen condensate pipe at ground level, bleed radiators, check the thermostat, timer and power, and reset the boiler once. Never take the casing off or touch anything on the gas side. That work must be done by a qualified gas engineer. If you smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999.
This tool uses artificial intelligence to suggest a likely cause based on the information you provide. It is not a professional diagnosis and does not replace an inspection by a qualified gas engineer. Results may be inaccurate or not apply to your boiler.
Never remove the boiler casing or work on any gas fitting yourself. If you smell gas or suspect carbon monoxide, call the National Gas Emergency Service on 0800 111 999 immediately.
Renzy accepts no liability for decisions made based on the output of this tool. This tool is intended for use in the United Kingdom only.