Vaillant F codes, explained properly
ecoTEC and ecoFIT boilers show an F code when they stop. Here is what each one means, which ones you can act on yourself, and which ones should not be reset twice.
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Reading a Vaillant F code
Vaillant uses an F followed by a number. The code is shown on the display and, on ecoTEC models, the fault history is stored on the board so an engineer can see what has been happening rather than relying on what the boiler is doing at that moment.
Two codes look similar and are not. F28 means the boiler never managed to light from cold. F29 means it lit and then lost the flame while running. The first usually points at ignition or supply, the second at something interrupting combustion mid cycle. Treating them as the same fault is how the wrong part gets fitted.
The reset rule, and why it matters
Almost every fault code can be cleared with the reset button, and almost everyone presses it more than once. That is the single most expensive habit in domestic heating.
A lockout is the boiler protecting itself. Resetting it tells the appliance to try again. Once is reasonable, because genuine one off glitches do happen. Repeatedly resetting a boiler that is locking out on ignition means it is pumping gas into the combustion chamber and failing to light it, over and over. Repeatedly resetting an overheat lockout means asking it to fire again into a system that cannot carry the heat away. Both do real damage, and both turn a known price repair into a bigger one.
Reset once. If it locks out again, stop and get it looked at.
Vaillant codes in detail
F22, low water pressure
The boiler has shut down because there is not enough water in the sealed system to run safely.
Why it happens: Water has either escaped or the expansion vessel has failed. A leak on pipework or a radiator valve, recently bled radiators, a pressure relief valve passing water outside, or a pressure sensor reading incorrectly.
What you can do: Repressurise to 1.0 to 1.5 bar using the filling loop, then note how long it holds. Days rather than months means there is a genuine fault to find.
What an engineer does: Checks the outside discharge pipe, tests the expansion vessel charge with the system drained down, and pressure tests to separate a leak from a vessel failure.
F28, ignition failure on startup
The boiler tried to light several times from cold and never established a flame, so it has locked out.
Why it happens: A frozen condensate pipe is the dominant cause between November and March. Otherwise: gas supply interrupted, a worn ignition electrode, a gas valve fault, or gas pressure too low at the appliance.
What you can do: Check the gas is on and other gas appliances work. Thaw an external condensate pipe with warm water, never boiling. Reset once only.
What an engineer does: Tests working gas pressure under load, which is the test that finds an undersized or partly blocked supply pipe. Checks the electrode, the ignition lead and the gas valve before condemning anything.
F29, flame lost during operation
The boiler lit correctly and then the flame went out while it was running. That is different from F28 and points somewhere different.
Why it happens: Something interrupting supply or combustion mid cycle. A recurring condensate blockage, a gas supply that drops under load, a flue restriction, or a gas valve that is not holding.
What you can do: Reset once. If it comes back, stop resetting, because a boiler repeatedly losing flame is a genuine combustion issue.
What an engineer does: Runs a flue gas analysis with the boiler at full rate, which shows combustion quality directly, and checks gas pressure while the appliance is working rather than idle.
F75, pump or pressure sensor fault
When the pump starts, the boiler expects to see a small change in pressure. It has not seen one, so it does not believe water is moving.
Why it happens: A seized or failed pump, a faulty pressure sensor, air trapped in the system, or sludge restricting circulation. F75 is the classic Vaillant code after a system has sat idle all summer.
What you can do: Check the pressure is correct first, because low pressure can bring this on. Beyond that it needs testing.
What an engineer does: Confirms whether the pump is actually turning, tests the sensor, and looks at the system water. If the water is black, the answer is a clean rather than a new pump, and fitting a pump into a dirty system just repeats the fault.
F27, flame detected when there should be none
The boiler is seeing a flame signal when the gas valve should be closed. This one is treated as urgent for good reason.
Why it happens: A faulty flame sensor, moisture on the control board, or a gas valve not sealing fully.
What you can do: Do not keep resetting this one. Turn the boiler off and call a Gas Safe engineer.
What an engineer does: Tests the gas valve for correct shut off and checks the flame sensing circuit and board for moisture damage. A valve that is not sealing is a safety matter, not a convenience one.
F20, overheat protection
The overheat thermostat has tripped because the appliance got hotter than it is allowed to.
Why it happens: Heat is not leaving the boiler fast enough. Pump failure, a blocked or sludged system, a stuck valve, or a failed overheat stat.
What you can do: Check whether radiators are getting hot at all, and check the pressure. Then leave it alone.
What an engineer does: Measures the temperature difference across flow and return, which immediately shows whether water is circulating, then works back to the cause rather than replacing the stat and hoping.
F62, gas valve shutoff delay
The board has measured the gas valve taking too long to close.
Why it happens: A worn or sticking gas valve, a wiring fault, or a board fault.
What you can do: Nothing safely. Anything involving a gas valve is engineer territory.
What an engineer does: Tests the valve response and the board output to establish which of the two is actually at fault, because replacing a gas valve unnecessarily is an expensive mistake.
F83, flow and return thermistor fault
The temperature sensors are not showing the change the boiler expects when it fires.
Why it happens: Often there is simply not enough water in the boiler, so nothing heats up as it should. Otherwise a failed thermistor or an air lock.
What you can do: Check the pressure gauge first, because a system that has emptied itself will produce this.
What an engineer does: Checks pressure and circulation before touching the sensors, then tests thermistor resistance against the manufacturer figures.
What these repairs cost
One fixed fee to attend and diagnose, then one fixed price for the repair, agreed before any part is ordered. Nothing runs on an hourly rate.
Thermistors and sensors are at the cheap end. Pumps and fans sit in the middle. Gas valves and control boards are the expensive ones, which is exactly why we test before replacing rather than working down a list. Our full price list is on the boiler repair cost page.
One thing worth knowing about F75 in particular: if the system water is black, fitting a new pump without cleaning the system will bring the same code back within months. That is a system cleaning job as much as a boiler one, and we will tell you when that is the case.
Vaillant questions we get asked
What people ring up about before we attend.
What is the difference between F28 and F29? +
My ecoTEC keeps showing F75. Do I need a new pump? +
Is F27 safe to reset? +
F22 keeps coming back after I top the pressure up. Why? +
Which Vaillant models do you repair? +
Do you cover my area? +
Other brands and what to do next
- Worcester Bosch fault codes
- Baxi fault codes
- Ideal boiler fault codes
- Navien fault codes
- Look any code up in the error code checker
- What a boiler repair actually costs
- Boiler repair across the North East
- Out of hours emergency cover
- Annual boiler servicing
Local pages: Sunderland, Durham, Washington, Chester le Street and Houghton le Spring.
There is a longer write up of one specific code here: the Vaillant F28 error code explained.