Vaillant F28 and F75 fault codes: what they mean and what to do

A Vaillant F28 code means your boiler tried to light, could not detect a flame and has locked out for safety. An F75 means the boiler could not sense a pressure change when its pump started, which usually points to low water pressure, air in the system, or a tired pump or pressure sensor. Both codes have a few safe checks you can do first. If those don't clear it, the fix is a job for a Gas Safe registered engineer.

Safety first If you can smell gas, don't touch the boiler or any electrical switches. Turn the gas off at the meter, open the windows and call the National Gas Emergency Service on 0800 111 999.

What does F28 mean on a Vaillant boiler?

F28 is an ignition lockout. Each time your boiler fires, it sparks and then checks for a flame. If it can't see one after several attempts, it stops trying and shows F28 so unburnt gas can't build up inside the boiler.

The usual causes are:

• No gas reaching the boiler. A prepayment meter out of credit, a gas valve knocked off at the meter, or a supply problem in the street.
• A frozen or blocked condensate pipe. This is the plastic pipe that carries waste water from the boiler to a drain outside. When it's blocked, the boiler can refuse to run.
• A worn ignition electrode or flame sensor. These parts wear with age and are often found during a service.
• A faulty gas valve or control board. Less common, and always a job for an engineer.

Checks you can do safely

1. Try your gas hob. If it won't light either, the problem is the gas supply, not the boiler.
2. If you have a prepayment meter, check the credit.
3. Check the handle on the gas valve by the meter. It should sit in line with the pipe.
4. In freezing weather, look at the condensate pipe outside. If it's frozen, pour warm (not boiling) water along it to thaw it.
5. Press the reset button once and give the boiler a few minutes.

Reset once, not over and over If F28 comes back after one reset, leave it there. Repeated resets won't fix the cause, and the code is telling you something inside the boiler needs attention.

If the gas supply is fine and the code returns, the fault is inside the boiler: the electrode, the gas valve or the control board. These sit behind the sealed casing, and only a Gas Safe registered engineer can legally work on them. Our boiler fault diagnosis service tests each part in turn to find the real cause before anything is replaced.

What does F75 mean on a Vaillant boiler?

A Vaillant F75 code means the boiler checked the system pressure as its pump started and saw no change. Your boiler uses a pressure sensor to confirm water is moving before it heats anything. With no signal, it stops to protect itself.

The usual causes are:

• Low system pressure, the most common cause by far
• Air trapped in the system or in the pump itself
• A failing pump that can no longer move water properly
• A faulty pressure sensor giving the boiler a false reading
• Sludge restricting water flow around the system

Checks you can do safely

1. Read the pressure on the boiler's gauge or display. With the heating off and the system cold, it should sit at around 1 to 1.5 bar.
2. If it's below 1 bar, top it up using the filling loop, following the instructions in your boiler manual.
3. If your radiators are cold at the top, bleed them, then check the pressure again as bleeding lowers it.
4. Press reset once.

Good to know Topping up fixes the code, not the cause. If the pressure drops again within days or weeks, water is escaping from the system somewhere. Our guide on why boilers keep losing pressure explains where to look.

If the pressure is correct and F75 still appears, the pump or pressure sensor is the likely culprit. Both are routine parts to replace as part of a boiler repair.

Why do fault codes keep coming back?

A code that returns is a symptom of something wearing out or a problem that hasn't been found yet. Electrodes wear, sludge builds up and expansion vessels lose their charge gradually, so faults tend to creep in rather than arrive overnight.

A yearly boiler service is the simplest way to catch these early. As an accredited Vaillant installer, we know these boilers well, and a service is often when a worn part shows itself before it causes a lockout on a cold evening.

When to stop and call an engineer

• The code returns after one reset
• You can smell gas
• Water is dripping from the boiler or its pipes
• The pressure keeps dropping after you top it up
• The fix means taking the boiler casing off

Frequently asked questions

Can I reset a Vaillant boiler showing F28?

Yes, once. Press the reset button and wait a few minutes. If the boiler lights and runs normally, the lockout may have been caused by a brief gas supply interruption. If F28 comes back, stop resetting and check your gas supply. If the gas is fine, the fault is inside the boiler and needs a Gas Safe registered engineer.

Is a Vaillant F75 fault dangerous?

F75 isn't dangerous in itself. It's a protective lockout that stops the boiler heating when it can't confirm water is circulating, which protects the boiler from damage. You'll have no heating or hot water until it's resolved. Low pressure is the most common cause and is often something you can top up yourself.

Can a frozen condensate pipe cause F28?

It can. When the condensate pipe freezes, waste water backs up inside the boiler and it may refuse to run, showing a lockout code such as F28. Pour warm (not boiling) water along the outside section of pipe until it thaws, then reset the boiler once. Insulating the outside section helps stop it happening again.

What is the difference between F28 and F29?

F28 means the boiler failed to light in the first place. F29 means it lit, then lost the flame while it was running. The causes overlap, including gas supply problems, a blocked condensate pipe or a worn flame sensor, but F29 can also point to low gas pressure or a flue problem. Both need an engineer if they return after one reset.

Still seeing F28 or F75?

If the checks above haven't cleared the code, give our team a ring on 01273 911551 and we'll talk you through what to do next. We're Gas Safe registered (568345) and work on Vaillant boilers across Brighton, Hove and surrounding Sussex towns. Book a Vaillant fault code diagnosis and we'll find the cause.