Ariston 6P1 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 6P1 fault code mean?
The 6P1 fault code on Ariston boilers signals that the air pressure switch (also called the differential pressure switch or monostat) failed to close its contacts when the boiler attempted to start. In practice, this means the boiler's control board could not confirm that the fan was generating adequate airflow to safely extract flue gases, so the appliance locked out as a precaution. A closely related code, 6P2, carries the same underlying fault but triggers during normal operation rather than at start-up — both codes point to the same components and are diagnosed and repaired in the same way.
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
- Damaged or melted Venturi tube Common
The Venturi tube creates the pressure differential that the air pressure switch measures. On Ariston boilers this is a well-documented failure point: if the boiler has been run with an air-locked or underfilled heating system, localised overheating can soften and distort the lower section of the Venturi tube. Even a small hole or deformation lets air bleed away, preventing the switch from seeing the correct pressure difference. The result is a 6P1 lockout every time the boiler tries to fire.
- Blocked or partially obstructed flue Common
If the flue outlet or intake is restricted — by a bird's nest, accumulated soot, a dislodged terminal cap, or ice during cold weather — the fan cannot move air freely. Reduced airflow means lower pressure at the switch, so the contacts never close. Inspecting the visible section of the flue terminal externally is the one check homeowners can safely carry out themselves.
- Faulty air pressure switch (monostat) Sometimes
The switch itself can fail mechanically or electrically: the diaphragm can split, the contacts can burn or corrode, or the rubber hoses connecting it to the Venturi tube can crack or become loose. Any of these prevent the switch from registering correct pressure and closing its contacts, regardless of whether the fan and flue are working perfectly.
- Fan failure or reduced fan output Sometimes
If the fan motor is beginning to fail, running slower than specified, or if its impeller is fouled with debris, it may not generate enough pressure to actuate the switch — even though it appears to be spinning. A fan that starts but underperforms is a less obvious fault than one that does not start at all, which is why 6P1 is not always immediately traced to the fan.
- PCB or wiring fault Rare
The PCB supplies power to the fan and monitors the air pressure switch signal. A damaged PCB, corroded connector, or broken wire in the circuit can cause the board to incorrectly report a switch-contact fault even when the switch itself is functioning normally. This is a less common cause but worth considering when other components check out correctly.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button (usually marked with a flame-and-arrow symbol) for approximately five seconds until the display clears, then allow the boiler to attempt a restart. If 6P1 returns immediately or within a short time, a single reset has ruled out a transient glitch and there is a genuine underlying fault. Do not repeatedly reset the boiler — two or three attempts at most — as persistent resets will not fix a mechanical or electrical fault.
- Check the flue terminal externally DIY safe
From outside your property, locate the flue outlet (typically a white plastic pipe exiting through an external wall or roof). Check visually that the terminal cap is present and unobstructed. In winter, look for ice build-up. Clear any obvious debris such as leaves or a bird's nest if you can reach it safely from ground level. Do not attempt to access roof-level flues or dismantle any pipework — call an engineer if the terminal is inaccessible or if you can see a significant blockage.
- Engineer inspection and testing of the Venturi tube Gas Safe engineer
A Gas Safe registered engineer will disconnect and remove the Venturi tube and inspect it for heat distortion, cracks, or small holes. In many 6P1 cases on Ariston boilers this component is found to be the root cause. If only dirty, it can be cleaned and cleared; if heat-damaged, it must be replaced before the boiler can operate safely.
- Engineer testing of the air pressure switch and hoses Gas Safe engineer
The engineer will check the rubber connection hoses for cracks or loose joints, then test the monostat with a multimeter to verify the contacts close at the correct pressure. A failed diaphragm or faulty contact will require the switch to be replaced. This is a straightforward parts-and-labour job in most cases.
- Engineer inspection of the fan Gas Safe engineer
The engineer will confirm the fan starts and reaches its correct operating speed, and inspect the impeller for debris or fouling. If the fan is underperforming or drawing excessive current, it will need to be replaced. Fan replacement on Ariston boilers is a Gas Safe engineer task because the appliance casing must be opened and the work verified before the boiler is returned to service.
- PCB diagnosis and replacement if required Gas Safe engineer
If all other components test within specification, the engineer will inspect the PCB for signs of heat damage, moisture ingress, or corroded connectors. A faulty board will need specialist diagnosis and, if confirmed faulty, replacement. Given the cost involved, an engineer may advise obtaining a quote for a new boiler — particularly if the appliance is over ten years old — before committing to a PCB replacement.
- Call a Gas Safe registered engineer Gas Safe engineer
If the fault persists after your external flue check and a boiler reset, contact a Gas Safe registered engineer to diagnose and repair the fault. You can verify any engineer's Gas Safe registration at gassaferegister.co.uk. Do not attempt to open the boiler casing, adjust the fan, or handle any gas components yourself.
Parts you may need
- Ariston air pressure switch (monostat) · from £55
- Ariston Venturi tube assembly · from £45
- Pressure switch connecting hoses/tubing kit · from £15
- Ariston fan assembly · from £90
- Ariston PCB (main circuit board) · from £220
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 £175–£320, depending on the underlying cause.
Frequently asked questions
What is the difference between Ariston fault codes 6P1 and 6P2?
Both codes indicate an air pressure switch contact delay, but they trigger at different points in the boiler's cycle. 6P1 appears when the switch fails to close before ignition — i.e., the fault is detected at start-up. 6P2 appears when the switch opens unexpectedly during normal running. Because both faults involve the same components — the Venturi tube, pressure switch, fan, and associated wiring — the diagnosis and repair process is identical for both codes.
Can I fix Ariston error 6P1 myself?
The only checks a homeowner can safely carry out are: resetting the boiler once or twice, and inspecting the external flue terminal for obvious blockages or ice. All internal work — removing the Venturi tube, testing the pressure switch, inspecting the fan, or touching the PCB — must be carried out by a Gas Safe registered engineer. Working on a gas appliance without Gas Safe registration is illegal in the UK and potentially dangerous.
How much does it cost to fix Ariston fault code 6P1 in the UK?
Most 6P1 repairs fall in the range of £175–£320, depending on which component is at fault. A Venturi tube or air pressure switch replacement — the two most common fixes — typically costs £175–£250 including parts and labour. Fan replacement tends to be slightly less if the fan itself is inexpensive, but labour can push the total to around £200–£300. A PCB replacement is significantly more expensive, often £400–£600, and at that price point it is worth comparing the cost of a new, more energy-efficient boiler with a full manufacturer's warranty.
Why does my Ariston boiler keep showing 6P1 after I reset it?
If the fault reappears quickly after each reset, the boiler has a genuine mechanical or electrical problem that a reset cannot cure. The most common culprit on Ariston boilers is a heat-damaged Venturi tube — often the result of the boiler having been run at low system pressure or with trapped air in the heating circuit. Other persistent causes include a split pressure-switch diaphragm or a fan that is running below its rated speed. An engineer will need to test each component in turn to pinpoint the fault.