Vaillant boilers display an F code when they detect a fault, and the codes are consistent enough across the ecoTEC and older ranges that most can be read without the manual. F22 is low water pressure, F28 and F29 are ignition or flame loss, F75 is the pressure sensor not seeing the pump run, F62 is a gas valve fault, F61 is a gas valve control fault and F72, F73 and F74 are sensor faults. F22 and a one-off F28 are things a homeowner can usually deal with. Almost everything else needs a Gas Safe engineer, and repeated resets on any of them do more harm than good.
This guide lists the codes engineers see most often on Vaillant boilers in Cornwall and Devon, what each one means in plain terms, and what to do about it.
F22: low water pressure
The system pressure has dropped below about 0.5 bar. Top it up to around 1.3 bar using the filling loop and the boiler should restart, sometimes after a reset. If it happens again within days, water is leaking somewhere or the expansion vessel is flat. Our guide to a boiler losing pressure explains the causes.
F28: ignition failed
The boiler tried to light and did not detect a flame. Common causes are a gas supply interruption, a frozen condensate pipe in winter, a faulty ignition electrode or lead, a gas valve fault or air in the gas pipe after work on the supply. Check the gas is on and the condensate pipe is not frozen, reset once, and if it comes back call an engineer.
F29: flame lost during operation
The boiler lit but the flame went out. Causes are similar to F28, plus a flue problem, a gust of wind or a condensate blockage. Again, one reset is reasonable; repeated F29 needs an engineer.
F75: pump or pressure sensor fault
The boiler ran the pump but the pressure sensor did not register a change in pressure. The classic causes are a sticking or failed pump, a faulty pressure sensor, air in the pump, or sludge in the system slowing the flow. F75 is one of the most searched Vaillant codes because it appears on ageing ecoTEC models as the pump wears and the sensor scales up. Bleeding the system and a reset occasionally clears it once; a persistent F75 means the sensor or pump needs replacing, and the system may need a flush.
F61, F62, F63, F64: gas valve and electronics
These are faults in the gas valve control, the gas valve switch-off or the boiler’s electronics. They are engineer-only, and a boiler showing them should not be reset repeatedly.
F72, F73, F74: temperature sensor faults
The flow and return sensors are giving readings the boiler cannot reconcile, or a sensor is short-circuited or disconnected. Sometimes a loose connector; often a failed sensor. An engineer replaces the sensor in a short visit.
F20: overheat
The boiler got too hot, usually because water is not circulating: a pump fault, a closed valve, air or sludge. Let the boiler cool, check the pressure and that radiator valves are open, reset once, and if it recurs call an engineer.
F27: flame detected with gas valve closed
A serious fault suggesting the gas valve is not closing or the flame sensor is faulty. Switch the boiler off and call an engineer.
F32 and F49: fan and bus faults
F32 is a fan fault, often a failing fan or a blocked flue. F49 is a communication fault between the boiler and its controls, sometimes a wiring or thermostat issue. Both need an engineer.
What you can do before calling
- Note the code and how often it appears.
- Check the pressure and top up if it is low.
- In winter, check the condensate pipe outside for ice.
- Check the gas supply is on and other gas appliances work.
- Reset once. If the code returns, stop.
Our guide to how to reset a boiler explains why one reset is the limit.
How DCI Boilers can help
DCI Boilers repairs Vaillant and every other major brand across Cornwall and Devon, with Gas Safe registered engineers who carry pumps, sensors, electrodes and fans for the common models and quote a fixed price before starting. Book boiler repairs, or if you have no heating and hot water, our emergency boiler call-out runs around the clock. Regular boiler servicing catches most of these faults before they lock the boiler out.
Need help now? Call DCI Boilers on 07888 862 603 or get a free quote.
Frequently asked questions
What does F75 mean on a Vaillant boiler?
The pump ran but the pressure sensor did not detect a pressure change. Usually a worn pump, a scaled sensor or sludge in the system.
Can I fix F22 myself?
Yes. Top the pressure up using the filling loop and reset. If it keeps returning, have the leak or expansion vessel checked.
Why does my Vaillant show F28 in winter?
A frozen condensate pipe is the commonest cause. Thaw it gently and reset once.
Is it safe to keep resetting a Vaillant boiler?
No. One reset per fault. Repeated resets on ignition or gas valve codes can cause damage and, on flue faults, a safety risk.