Viessmann F5 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann F5 fault code mean?
The F5 code appears on Viessmann boilers when the PCB cannot confirm that the air pressure switch is functioning correctly. The air pressure switch monitors airflow through the boiler — it tells the PCB that the fan is running and that conditions are safe before allowing the burner to fire. When the switch fails to send the expected signal, the boiler locks out as a precaution against incomplete combustion, which can produce carbon monoxide. In some cases the root cause isn't the switch itself but a blocked burner, a jammed mixture damper (on older Viessmann models), a frozen condensate pipe, or a disrupted gas supply — all of which can prevent the switch from closing or opening as the boiler expects.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or stuck air pressure switch Common
The pressure switch itself can fail mechanically or electrically. The diaphragm inside the switch can split or stick, or the electrical contacts can corrode, meaning the correct signal never reaches the PCB. A Gas Safe engineer can test it with a multimeter and replace it if needed.
- Blocked or frozen condensate pipe Common
Viessmann condensate boilers produce acidic condensate that drains away through a plastic pipe, often routed externally. In cold weather this pipe can freeze solid, backing up inside the boiler and disrupting operation — sometimes triggering an F5 lockout rather than the more typical condensate-specific code. If the fault appeared during a cold snap, this is worth investigating first.
- Gas supply problem Sometimes
If the mains gas pressure is low or interrupted — for example because a gas meter regulator has frozen in cold weather or a supply issue affects your area — the boiler cannot build sufficient pressure for the switch to confirm safe firing conditions. Check that other gas appliances in your home are working normally.
- Jammed mixture damper (older models) Sometimes
Some older Viessmann models use a small metal flap called a mixture damper at the fan outlet. It opens under fan pressure and closes under gravity when the fan stops. If it becomes stuck closed — due to corrosion, debris, or a seized pivot — the air pressure switch won't see the airflow it expects and the boiler locks out with F5.
- Faulty gas valve Sometimes
A gas valve that sticks in the closed position will prevent fuel from reaching the burner. Without gas flow the combustion conditions the pressure switch expects to detect never materialise, and the boiler shuts down. Gas valve work must only be carried out by a Gas Safe registered engineer.
- Blocked or dirty burner Sometimes
A burner clogged with debris, rust particles, or combustion residue can restrict gas flow and produce an unstable or weak flame. This can cause the pressure switch to report unexpected readings, triggering an F5 lockout.
- PCB fault Rare
The PCB interprets the signal from the air pressure switch. If the board itself has developed a fault — through moisture ingress from an internal leak, a loose connector, or component failure over time — it may log an F5 even when the switch is perfectly healthy. PCB issues are a less common but possible cause, particularly on older boilers.
How to fix it
- Check your gas supply is working DIY safe
Turn on a gas hob or check another gas appliance. If nothing gas-powered is working in your home, the issue is with your gas supply rather than the boiler. Contact your gas supplier or call 0800 111 999 (National Gas Emergency Service) if you suspect a supply failure or smell gas.
- Inspect the external condensate pipe for freezing DIY safe
Locate the white plastic condensate pipe — usually on an outside wall. If it feels solid or you can see ice, thaw it gently by pouring warm (not boiling) water along its length from top to bottom. Never use a naked flame. Once it flows freely, move on to the reset step.
- Reset the boiler (up to twice) DIY safe
Once you've addressed anything visible (e.g. a thawed condensate pipe), press and hold the reset button for approximately 2 seconds. If the boiler locks out again immediately, do not reset more than twice — repeated resets without resolving the underlying fault can cause further damage and mask the true problem.
- Have a Gas Safe engineer test the air pressure switch Gas Safe engineer
A registered engineer will use a multimeter to check the switch's electrical continuity and may use a manometer to verify the pressure differential at which it operates. If the switch is faulty, it will be replaced — a straightforward repair when diagnosed correctly.
- Have the engineer inspect the mixture damper (older models) Gas Safe engineer
On applicable older Viessmann models the engineer will check the damper flap moves freely when the fan runs. If it is seized, it may be freed and reseated, or replaced entirely depending on its condition.
- Have the engineer test gas pressure and inspect the gas valve Gas Safe engineer
The engineer will measure gas pressure at the meter, at the boiler inlet, and across the valve itself. A valve that is not opening correctly will be replaced — this is a gas-carrying component and must only be touched by a Gas Safe registered engineer.
- Have the engineer inspect and clean the burner Gas Safe engineer
If the burner is found to be dirty or partially blocked, the engineer will carry out a controlled clean or replace the burner assembly as appropriate. This is part of a boiler service and may be combined with other checks.
- Have the PCB tested and replaced if necessary Gas Safe engineer
If all other components test correctly, the engineer will inspect the PCB for signs of moisture damage, burnt tracks, or loose connectors. A faulty PCB will need to be replaced — this is one of the more expensive repairs, so the engineer should confirm the diagnosis thoroughly before ordering parts.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If your own checks haven't resolved the fault, or if the boiler locks out again after resetting, you need a Gas Safe registered engineer to carry out a full diagnostic. You can verify an engineer's registration at gassaferegister.co.uk.
Parts you may need
- Air pressure switch · from £45
- Mixture damper assembly (older models) · from £25
- Gas valve · from £120
- PCB (printed circuit board) · from £220
- Condensate trap · from £20
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£350, depending on the underlying cause.
Frequently asked questions
Can I reset a Viessmann F5 fault myself?
Yes, you can attempt a reset — press and hold the reset button for around 2 seconds. If the boiler fires up and runs normally, monitor it over the next hour or so. However, if it locks out again with F5, do not keep resetting it. Repeated resets without fixing the underlying fault won't help and can in some cases cause additional wear. Two unsuccessful resets should be your limit before calling a Gas Safe engineer.
Could a frozen condensate pipe really cause an F5 code?
Yes, though it's not the most obvious cause. When the condensate pipe freezes, the acidic water it carries backs up inside the boiler and can interfere with sensors and pressure components in unexpected ways, sometimes displaying as F5 rather than a condensate-specific code. If your boiler showed F5 during a cold snap, thawing the external condensate pipe with warm water is a sensible free first step before calling anyone out.
How much does it cost to fix a Viessmann F5 fault in the UK?
Most F5 repairs fall between £100 and £350 including labour. Replacing the air pressure switch is the most common fix and typically costs under £150 all-in. A gas valve replacement generally runs to around £250–£300. PCB replacement is less common as an F5 cause but can cost £350–£500 or more depending on the model — if that figure is quoted on a boiler over ten years old, it's worth comparing it against the cost of a new boiler.
Is the Viessmann F5 fault dangerous?
The F5 lockout itself is a safety feature — the boiler has shut itself down because it cannot verify safe operating conditions. You should not attempt to bypass the lockout or continually reset the boiler without finding the cause. If you smell gas at any point, do not reset the boiler, leave the property, and call the National Gas Emergency Service on 0800 111 999 immediately.