Ariston 608 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 608 fault code mean?
Fault code 608 indicates that the boiler's fan has started up as normal, but the air pressure switch has failed to close to confirm that the flue is clear and combustion gases are being safely expelled. Because the boiler cannot verify safe ventilation, it treats this as a combustion safety issue and locks out to protect the household. In plain terms: the fan is spinning, but the component that checks whether air is actually moving through the system hasn't responded correctly. The boiler will not fire until this is resolved. Related air pressure switch faults on some Ariston models may also appear as codes 6P1 or 6P2, which can point to a similar root cause — particularly a damaged venturi tube causing a small air leak that prevents the switch from operating.
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
- Blocked, kinked, or split pressure switch hose Common
A rubber tube connects the fan housing to the air pressure switch. If this tube has split, kinked, or become blocked with debris or condensate, the switch receives no pressure signal even though the fan is running. This is one of the most common and cheapest causes of code 608 to fix.
- Failed air pressure switch Common
If the hose is intact and properly connected, the air pressure switch itself may have failed mechanically or electrically and is no longer able to close its contacts when it receives the pressure signal from the fan. Switches can fail gradually due to age or condensate ingress.
- Wiring or connection fault between switch and PCB Sometimes
Loose, corroded, or damaged wiring between the air pressure switch and the PCB can prevent the signal from reaching the control board, causing the boiler to behave as if the switch has not closed even when it physically has. This can be intermittent and harder to diagnose.
- Damaged or melted venturi tube Sometimes
On some Ariston models, overheating can cause the venturi tube inside the boiler to warp or melt slightly, creating a small air leak in the pressure circuit. This prevents the switch from building enough differential pressure to close. Related codes 6P1 and 6P2 may also appear alongside 608 in this scenario.
- PCB fault Rare
In a small number of cases, the PCB itself misreads or fails to register the pressure switch signal even when all other components are working correctly. PCB faults are less common but should be considered after all other causes have been ruled out by a Gas Safe engineer.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button for approximately 5 seconds until the boiler attempts to restart. A single reset is reasonable — if code 608 returns immediately or after a short time, do not keep resetting. Repeated resets on a combustion safety fault can mask an underlying problem and are not recommended.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home (such as a gas hob) are working normally. If there is no gas supply to the property, contact your gas supplier rather than an engineer.
- Visually inspect the pressure switch hose for obvious damage DIY safe
Locate the rubber hose running from the fan assembly to the air pressure switch — it is usually a short, thin tube visible inside the boiler casing. Check whether it has visibly split, disconnected at either end, or become kinked. Do not touch any gas connections or components. If you can see clear damage to the hose, report this to the engineer so they can bring the right part. Do not attempt to repair or replace it yourself.
- Check that the flue terminal outside is unobstructed DIY safe
Go outside and look at where the flue exits the property (usually a white plastic terminal on an external wall). Make sure it is not blocked by debris, a bird's nest, ice, or anything else. Do not attempt to dismantle the flue — visual inspection only.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
If the fault persists after a single reset, you need a Gas Safe registered engineer. They will test the air pressure switch electrically, inspect and replace the pressure switch hose if damaged, check the venturi tube for heat damage, verify fan operation and output, and test wiring continuity back to the PCB. Only a Gas Safe registered engineer is legally permitted to carry out this work on a gas appliance in the UK.
Parts you may need
- Air pressure switch hose (rubber tube) · from £15
- Air pressure switch · from £45
- Boiler fan / flue fan assembly · from £120
- Venturi tube · from £35
- PCB (printed circuit board) · from £200
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–£300, depending on the underlying cause.
Frequently asked questions
Is Ariston fault code 608 dangerous?
Code 608 is a combustion safety lockout, which means the boiler has shut itself down because it cannot confirm that flue gases are being safely removed. You should not attempt to keep the boiler running or reset it repeatedly. If you smell gas or fumes at any point, do not operate any switches — leave the property and call the National Gas Emergency line on 0800 111 999 from outside. Otherwise, the boiler is safe in its locked-out state; simply arrange for a Gas Safe engineer to inspect it.
What is the most common fix for code 608 on an Ariston boiler?
The most frequent cause is a split or blocked rubber pressure switch hose — the short tube that connects the fan to the air pressure switch. This is an inexpensive part (typically under £20) and relatively quick for an engineer to replace, making the total repair cost fairly modest. If the hose is fine, the air pressure switch itself is the next most likely culprit and is also a straightforward repair. More involved repairs involving the fan, venturi tube, or PCB cost more but are less common.
Could the PCB be the problem even though the code points to the pressure switch?
Yes, though it is relatively rare. Ariston fault codes are not always a precise diagnosis — code 608 tells the engineer where to look first, not necessarily what the definitive fault is. If the pressure switch, hose, and wiring all test correctly, the PCB may be misreading or failing to register the switch signal. PCB replacement is significantly more expensive (parts alone can be £200 or more, with total costs potentially reaching £400–£500), so a competent engineer will rule out all other causes before recommending that route.
Does Ariston fault code 608 apply to all Ariston boiler models?
Ariston uses a consistent fault code system across most of their boiler range, so code 608 carries the same meaning on the Clas, Genus, E-Combi, E-System, Alteas, and Egis ranges. If your model displays related codes such as 6P1 or 6P2 alongside or instead of 608, these also point to air pressure switch circuit faults and may specifically suggest a damaged venturi tube — mention the full code displayed to your engineer so they can investigate accordingly.