We find leaks, and F28 is not one
You will find plenty of leak detection firms happy to take a booking for any boiler fault code. We would rather tell you now: F28 is almost never a leak, and a leak detection visit will not clear it. What follows is what the code means and who to call, because sending you to the right trade first time is worth more to us than a wasted callout.
Vaillant defines F28 as "The flame signal was not detected during the ignition phase". Their engineer documentation puts it as "Start-up failure: Ignit. unsuccessful", with the cause recorded as failure during start-up or ignition unsuccessful, or the gas pressure monitor or thermal cut-out having been triggered. In plain terms: the boiler asked for gas, opened the valve, sparked, and the flame sensor saw nothing. After a set number of attempts it locks out rather than keep releasing gas.
The three causes worth checking before you call anyone
1. A frozen condensate pipe, in cold weather
This is the one you can often fix yourself, and it is the most common cause in a British winter. A condensing boiler produces acidic condensate that drains through a plastic pipe, usually to an outside drain. When that pipe freezes, the condensate backs up, the boiler detects the blockage and refuses to fire.
Vaillant's own guidance: the pipe is usually frozen at its most exposed external point, typically the last stretch just above the drain. Pour warm water along the affected section until the ice clears, and explicitly do not use boiling water from a kettle, which can crack the pipe. Reset the boiler afterwards, and lag the external run with pipe insulation so it does not happen again next cold snap.
How to know this is your cause: the fault appeared during a cold spell, particularly overnight or first thing in the morning, and there may be a gurgling from the boiler as it tries to fire.
2. No gas reaching the boiler
Check, in this order: is the gas stopcock fully open; is the hob lighting; if you are on a prepayment meter, is there credit; has there been a supply interruption in your street. A boiler with no gas will spark, see no flame and throw F28 exactly as if something were broken. Vaillant's advice is the same — if the gas is on and working, it is time to call a professional.
3. Everything else, which is inside the boiler
Vaillant's fault table sends the engineer to check the gas stopcock, gas flow pressure, the gas valve assembly, the air intake pipe for blockage or a loosened screw, the condensate route for blockage, the multiple plug and cable harness, the ignition transformer, cable, plug and electrode, the monitoring electrode, the electronics, the earthing and the CO₂ setting. Electrodes coke up, ignition leads perish, and gas valves drift out of adjustment. All of it is behind the case.