Chaffoteaux 6P4 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 6P4 fault code mean?
The 6P4 code sits within the 6Px family of faults on Chaffoteaux boilers, all of which relate to the fan and air pressure switch (APS) circuit. When 6P4 appears, the boiler has detected that the fan started as expected but the air pressure switch failed to confirm adequate flue draught within the required time window. As a safety measure, the boiler locks out before firing to prevent combustion gases — including carbon monoxide — building up inside the heat exchanger or escaping into the property. Related codes in the same family include 6P1 and 6P2 (contact closure delay on the air pressure switch), and codes 607 and 608 (APS tripped before ignition, and APS not responding despite fan running respectively). All point to the same fan–APS circuit and are diagnosed in much 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
- Blocked or partially obstructed flue terminal Common
Debris such as a bird nest, leaves, or ice at the external flue terminal restricts airflow so the fan cannot generate the pressure differential the APS needs to close. This is one of the most frequently diagnosed causes of the 6Px code family.
- Fan running slowly or beginning to fail Common
If the fan motor is worn or the impeller is coated with dust and deposits, fan speed drops. The reduced draught is insufficient to trip the air pressure switch, even though the fan appears to be running. The boiler locks out before any gas is released.
- Faulty or stuck air pressure switch Common
The APS diaphragm can rupture or the switch contacts can become corroded or stuck. A failed diaphragm means pressure changes are not transmitted to the switch contacts, sending a false or absent signal to the PCB regardless of how well the fan is performing.
- Damaged, kinked, or leaking rubber sensing hose Sometimes
A small rubber tube connects the fan or venturi to the APS. If this hose cracks, splits, or becomes blocked with condensation deposits, the pressure signal never reaches the switch. Even a slight kink can cause intermittent faults that are difficult to reproduce.
- Wind blowback at the flue terminal Sometimes
Strong or gusting wind blowing directly into the flue terminal can temporarily reverse the pressure across the APS, causing it to trip even when the fan and flue are perfectly healthy. Faults that appear only in windy conditions and then clear on reset point strongly to this cause.
- Damaged venturi tube from prior overheating Rare
If the boiler was ever fired with an underfilled heating system or left dormant for a long period, localised overheating can partially melt the venturi tube. Even a small hole allows air to be drawn in outside the normal path, robbing the APS of the pressure signal it expects.
- Moisture ingress into the APS or wiring Rare
On older Chaffoteaux boilers, condensation can work its way into the air pressure switch body or its wiring connector, causing high-resistance connections or intermittent contact failures that mimic a genuine fan or flue fault.
How to fix it
- Check the flue terminal from outside DIY safe
Go outside and look at the point where the flue pipe exits the wall or roof. Look for any obvious blockage — a bird nest, ice plug, crushed terminal, or heavy deposit of debris. Do not attempt to dismantle the terminal yourself, but note what you see so you can report it to an engineer.
- In cold weather, check the condensate pipe DIY safe
Locate the white plastic condensate drain pipe that exits through an outside wall, usually near the boiler. If it feels solid or has visible ice around it, it may be frozen. Gently pour warm (not boiling) water along the pipe to thaw it, then attempt a reset. A frozen condensate pipe can sometimes trigger related lockouts.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on the boiler's front panel as described in your user manual. Wait at least 60 seconds after the boiler restarts to see whether it lights and runs normally. If it locks out again with 6P4, do not reset more than two or three times in total — repeated resets without resolving the underlying fault can mask a safety issue.
- Have a Gas Safe engineer inspect and test the fan Gas Safe engineer
The engineer will run the boiler in diagnostic mode to measure fan speed and current draw. The fan impeller will be cleaned of any dust or deposits and the bearings checked for wear. If the fan motor is failing or cannot reach the correct speed, it will need to be replaced.
- Have the air pressure switch tested and replaced if necessary Gas Safe engineer
The engineer will test APS resistance, check the diaphragm integrity, and inspect the rubber sensing hose for cracks, blockages, or kinks. Temporarily pinching the tube and observing whether pressure holds can confirm whether the diaphragm has ruptured. A faulty switch or hose will be replaced.
- Have the flue and air intake thoroughly checked Gas Safe engineer
The engineer will inspect the full flue run internally as well as the terminal. Soot build-up, ice formation at bends, or a partially collapsed liner can all restrict flow without being visible from outside. They will also check for correct flue gradient to avoid condensate pooling.
- Have the venturi tube and PCB wiring inspected Gas Safe engineer
If no obvious fault is found in the fan or APS, the engineer should inspect the venturi tube for heat damage and check the APS wiring loom and PCB connector for corrosion or loose pins. PCB replacement is only indicated if wiring checks out clean and the fault persists.
- Call a Gas Safe registered engineer if the fault persists or recurs Gas Safe engineer
If 6P4 returns after a reset or the boiler will not stay running, this is not a fault to leave. The air pressure switch is a critical safety device. Contact a Gas Safe registered engineer, quote the exact code and your boiler model, and arrange an inspection as soon as possible.
Parts you may need
- Air pressure switch (APS) · from £45
- APS rubber sensing hose / tubing kit · from £12
- Fan assembly (motor and impeller) · from £120
- Venturi tube · from £25
- Flue terminal / terminal guard · 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 £150–£380, depending on the underlying cause.
Frequently asked questions
Is the Chaffoteaux 6P4 code dangerous — can I keep using the boiler?
You should not continue running a boiler that is locking out with 6P4. The air pressure switch is a key safety device that prevents the boiler firing when flue gases cannot be safely expelled. If it is not working correctly, harmful gases including carbon monoxide could accumulate. Switch the boiler off, do not repeatedly reset it, and arrange for a Gas Safe engineer to inspect it promptly.
Can wind cause the 6P4 code to appear even when nothing is broken?
Yes. Strong or gusting wind blowing directly into the flue terminal can temporarily reverse the pressure the air pressure switch senses, causing a lockout. If your 6P4 code only appears during very windy conditions and the boiler resets and runs normally afterwards, tell your engineer — they may recommend a wind-deflector terminal or a change in flue orientation.
How much does it typically cost to fix a Chaffoteaux 6P4 fault in the UK?
Most owners pay between £150 and £380 including parts and labour. Replacing the air pressure switch or its sensing hose sits at the lower end of that range. A full fan assembly replacement typically costs more. In rare cases where the PCB is also found to be faulty, costs can rise beyond £400–£500, but PCB faults are an uncommon cause of this specific code and should only be diagnosed after the fan and APS have been ruled out.
What is the difference between fault codes 6P1, 6P2, 607, 608, and 6P4 on a Chaffoteaux boiler?
They all belong to the same fan and air pressure switch family but describe slightly different failure points. Codes 6P1 and 6P2 indicate a delay in the APS contact closing — the switch is responding but too slowly. Codes 607 and 608 relate to the APS tripping before ignition begins (607) or the APS not responding at all despite the fan running (608). Code 6P4 specifically means the fan started but the APS did not confirm adequate airflow within the required timeframe. All of these point an engineer towards the same components: the fan, the sensing hose, and the air pressure switch itself.