Chaffoteaux 620 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 620 fault code mean?
Fault code 620 on a Chaffoteaux boiler indicates a problem within the fan and flue airflow system. The boiler's control board has detected that the fan and air pressure switch are not communicating correctly — for example, the fan may be running but failing to generate the expected pressure change that triggers the air pressure switch, or a flue-related sensor is returning an out-of-range reading. As a safety precaution the boiler locks out to prevent combustion gases from building up. Code 620 sits within Chaffoteaux's 600-series fault family, which covers all flue gas, fan, and airflow-related issues. Related codes in the same family include 601 (flue gas sensor anomaly), 610 (thermal fuse open, typically due to overheating), and 612 (general fan anomaly). If your display shows one of those instead, the diagnostic process is very similar to what is described here.
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 seized fan motor Common
The fan is responsible for pushing combustion gases safely out through the flue. If the motor is worn, seized, or running below the correct speed, the air pressure switch will never receive the pressure signal it needs and the boiler locks out. Fan failure is the single most common cause of a 620 fault on Chaffoteaux boilers.
- Failed or stuck air pressure switch Common
The air pressure switch monitors the airflow created by the fan. If the diaphragm inside the switch splits or the switch sticks, it can fail to confirm adequate airflow even when the fan is working normally. Equally, a switch stuck in the closed position when the fan is off will also trigger a lockout.
- Blocked or kinked pressure switch tubing Common
A small rubber or plastic tube connects the fan housing to the air pressure switch. If this tube develops a crack, becomes kinked, or one of its push-fit connections loosens, air pressure is lost before it reaches the switch. The switch then reports no airflow even though the fan is spinning correctly.
- Blocked or obstructed flue terminal Sometimes
The external flue outlet can become partially or fully blocked by bird nests, wind-blown debris, ice build-up in winter, or even a neighbouring structure. Restricted airflow raises back-pressure and prevents the system from operating safely, causing the boiler to shut down.
- Faulty flue NTC temperature sensor Sometimes
The 600-series fault codes on Chaffoteaux boilers also cover flue sensor faults. If the NTC sensor on the flue is giving readings outside the expected range — perhaps due to a wiring break or sensor degradation — the PCB may interpret this as an airflow anomaly and issue a 620 lockout.
- Wind-induced false triggering Sometimes
Strong or gusting wind blowing directly into the flue terminal can momentarily reverse airflow and cause the pressure switch to trip. The fault may clear after a single reset and not return, or it may recur each time wind conditions are similar. This is more common with older or poorly positioned flue terminals.
- Wiring fault or PCB failure Rare
Damaged wiring to the fan or pressure switch — loose connectors, corroded terminals, or chafed cables — can mimic the electrical signatures of a component failure. In rarer cases, the PCB itself develops a fault that causes it to misinterpret fan or pressure switch signals. A PCB fault should only be diagnosed after all other causes have been ruled out.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on your Chaffoteaux boiler (usually marked with a flame or reset symbol on the fascia) and hold it for the required time as shown in your user manual. Wait a full two minutes for the boiler to attempt a restart. If it fires up and runs normally, monitor it closely over the next day or two. If the fault returns, do not keep resetting — repeated resets without fixing the underlying cause can cause further damage.
- Check your gas supply DIY safe
Make sure the gas isolation valve on the supply pipe to the boiler is fully open (handle in line with the pipe). Then check whether other gas appliances in the property — the hob, for instance — are working normally. If no gas appliances are working, contact your gas supplier rather than an engineer.
- Inspect the external flue terminal for obstructions DIY safe
Go outside and look at the flue outlet on the wall or roof. Check for anything that could be restricting airflow: bird nests, leaves, ice, or damage to the terminal cowl. If you can safely reach the terminal and there is a clear physical obstruction such as a nest or ice, remove it carefully. Do not probe inside the flue or attempt to remove the terminal itself.
- Check the condensate pipe in cold weather DIY safe
In freezing temperatures, the external section of the white plastic condensate pipe can freeze solid, which can cause related boiler lockouts. If you suspect this, gently pour warm (not boiling) water over the external section of the pipe until it flows freely, then reset the boiler. Note that a frozen condensate pipe typically triggers its own specific fault code, but it is worth ruling out.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
All internal inspection and repair work on a Chaffoteaux boiler must be carried out by a Gas Safe registered engineer. The engineer will test the fan motor and measure its speed, inspect the air pressure switch and replace it if the diaphragm has failed, check the pressure switch tubing for cracks or blockages, test the flue NTC sensor, inspect wiring connections to the fan and PCB, and clear or replace any faulty components. Ask to see the engineer's Gas Safe ID card before work begins. You can verify registration at gassaferegister.co.uk.
Parts you may need
- Fan motor assembly (Chaffoteaux compatible) · from £110
- Air pressure switch · from £35
- Pressure switch connecting tube/hose · from £12
- Flue NTC temperature sensor · from £28
- PCB (printed circuit board) · from £180
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 £130–£380, depending on the underlying cause.
Frequently asked questions
Is fault code 620 dangerous — should I leave the house?
The boiler has locked itself out as a safety measure, so combustion is not taking place and there is no immediate danger from carbon monoxide while the boiler is in lockout. You do not need to evacuate. However, you should not attempt to override the lockout or keep resetting the boiler repeatedly, as this could bypass safety systems. Get it looked at by a Gas Safe engineer promptly — do not leave it weeks. If you smell gas at any point, leave the property immediately, do not operate any switches, and call the National Gas Emergency Service on 0800 111 999.
Can I fix a Chaffoteaux 620 fault myself?
The only checks a homeowner can safely carry out are: a single reset, confirming the gas supply is on, looking at the external flue terminal for obvious blockages, and thawing a frozen condensate pipe in cold weather. Everything else — testing the fan, replacing the air pressure switch, checking sensor wiring, or touching the PCB — must be done by a Gas Safe registered engineer. Attempting internal boiler repairs without Gas Safe registration is illegal under UK gas safety regulations and invalidates your home insurance.
How much does it typically cost to fix a Chaffoteaux 620 fault?
For most homeowners the repair falls between £130 and £380 including parts and labour. A straightforward air pressure switch or sensor replacement sits toward the lower end, while a full fan motor replacement with a standard call-out tends to land in the £200–£350 range. A PCB replacement is less common but significantly more expensive — typically £350–£550 including labour — and should only be confirmed as necessary after the fan, switch, tubing, and wiring have all been tested and found to be in order. Engineers in London and the South East generally charge 10–15% more than the national average.
My Chaffoteaux shows code 620 only on cold or windy days — is that normal?
This is a recognised pattern. Strong wind blowing directly into the flue terminal can momentarily reverse the airflow and cause the air pressure switch to trip, triggering a 620 lockout. Similarly, in very cold weather, a partially blocked or poorly insulated flue can restrict airflow enough to trigger the fault. A single reset may clear it temporarily, but recurring weather-related faults usually indicate that the flue terminal cowl is worn or poorly sited, or that an underlying component such as the fan or pressure switch is marginal and only fails under adverse conditions. An engineer can assess whether the terminal needs replacing or repositioning.
What is the difference between codes 601, 610, 612, and 620 on a Chaffoteaux boiler?
All four codes belong to the 600-series, which covers the flue gas and airflow system. Code 601 points specifically to a fault with the flue gas sensor (the NTC that monitors exhaust temperature). Code 610 indicates the thermal fuse has blown, usually because a component overheated. Code 612 is a general fan anomaly — the PCB has detected the fan is not operating correctly. Code 620 is most closely associated with the interaction between the fan and the air pressure switch — the fan may be running but not generating the expected airflow signal, or the switch itself has failed. In practice there is some overlap in symptoms, and a Gas Safe engineer will test all these components together to pinpoint the exact cause.