Chaffoteaux 601 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 601 fault code mean?
Fault code 601 on a Chaffoteaux boiler indicates that the boiler's safety system has detected a problem with exhaust gas evacuation. The boiler monitors this via a flue gas sensor or air pressure switch — a safety device that confirms combustion gases are being properly expelled through the flue before allowing ignition. When this sensor fails to detect normal draught conditions, the boiler enters a safety lockout to prevent the risk of harmful gases — including carbon monoxide — entering your home. The boiler will attempt to restart after approximately 12 minutes; if conditions return to normal it may fire successfully, but if the fault persists it will lock out again. Because this fault directly involves flue gas safety, a Gas Safe registered engineer should be involved in diagnosis and repair.
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 obstructed flue Common
The most frequently reported cause. The external flue terminal can become partially or fully blocked by bird nests, accumulated debris, leaves, or ice in cold weather. Even a partial blockage reduces draught sufficiently to trip the pressure switch. Field engineers report that blocked or dirty flue systems account for the majority of 601 call-outs, often linked to a lack of annual servicing.
- Faulty or clogged air pressure switch Common
The air pressure switch contains a rubber diaphragm that responds to the negative pressure created by the fan. Over time the diaphragm can perforate, the internal chamber can become fouled with condensate deposits, or the electrical contacts can oxidise. Any of these will cause the switch to send an incorrect signal to the PCB, triggering code 601 even when the flue is perfectly clear.
- Failing or dirty flue fan (draught inducer) Sometimes
The fan creates the negative pressure that pulls combustion gases out through the flue. If the fan blades are caked in dust or combustion residue, or if the motor is weakening due to worn bearings or a failing run capacitor, the airflow it produces drops below the threshold needed to close the pressure switch. The boiler then cannot confirm safe draught and locks out.
- Wiring or electrical connection fault Sometimes
Loose, corroded, or damaged wiring between the air pressure switch and the PCB can cause intermittent or permanent loss of signal. The boiler then behaves as though no draught is present, producing a sporadic or persistent 601 fault even when the physical components are in good order.
How to fix it
- Turn the boiler off at its external isolator switch and do not attempt to relight it immediately DIY safe
If you suspect a flue or gas issue, your first priority is safety. Switch the boiler off at the wall isolator. Open windows if you feel unwell or smell gas. Do not use any electrical switches if you suspect a gas leak — call the National Gas Emergency Service on 0800 111 999 immediately.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home (e.g. a gas hob) are working normally. If there is no gas supply to your property, contact your gas supplier rather than resetting the boiler.
- Visually inspect the external flue terminal from outside — but do not attempt to clear it yourself DIY safe
Go outside and look at the point where the flue exits the building. Check for obvious obstructions such as a bird nest, large accumulation of debris, or ice blocking the terminal. You can note what you see, but do not attempt to unblock or modify the flue — this must be done by a Gas Safe engineer.
- Reset the boiler once DIY safe
If the flue terminal appears clear and there is no smell of gas, press the reset button on the boiler once and wait for it to attempt a restart. If the boiler fires up and runs normally, monitor it over the next few hours. If fault code 601 returns, do not continue resetting — repeated resets on a genuine flue fault can be dangerous.
- Do not reset the boiler more than once or twice — call a Gas Safe registered engineer Gas Safe engineer
If code 601 returns after a single reset attempt, the underlying fault needs professional diagnosis. A Gas Safe registered engineer will inspect the flue system for blockages or damage, test and if necessary replace the air pressure switch, assess the fan for performance and mechanical wear, and check all associated wiring and PCB connections. Do not attempt to inspect or replace any of these components yourself.
Parts you may need
- Air pressure switch / draught pressure switch · from £45
- Flue fan / draught inducer motor · from £120
- Flue terminal kit (horizontal) · from £35
- Fan wiring harness / connector · 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–£320, depending on the underlying cause.
Frequently asked questions
Is Chaffoteaux fault code 601 dangerous?
It can be. Code 601 means the boiler's safety system could not confirm that combustion gases were being safely expelled through the flue. If the fault is genuine — rather than a one-off sensor glitch — there is a risk that harmful gases including carbon monoxide could accumulate. If you or anyone in the property feels unwell, has a headache, or feels dizzy, evacuate immediately and call 999. Fit a carbon monoxide alarm if you do not already have one, and call a Gas Safe engineer before using the boiler again.
Can I fix Chaffoteaux fault code 601 myself?
Homeowners can safely check that their gas supply is on, visually look at the external flue terminal for obvious blockages, and attempt a single reset. Beyond that, all diagnosis and repair — including clearing the flue, testing or replacing the pressure switch, and inspecting the fan — must be carried out by a Gas Safe registered engineer. Working on flue components without the correct qualifications is illegal and potentially life-threatening.
How much does it cost to repair a Chaffoteaux 601 fault?
Most repairs for code 601 fall in the region of £100 to £320, depending on what the engineer finds. A flue clean or pressure switch replacement is typically at the lower end; fan replacement usually costs £225–£275 including parts and labour. If the fault turns out to involve the PCB, costs can rise significantly above this range — in that situation, and especially if your boiler is over 10–12 years old, it may be worth comparing repair costs against a new boiler installation.
Why does my Chaffoteaux boiler keep showing code 601 and then clearing itself?
This intermittent pattern — where 601 appears, the boiler eventually restarts, then the fault returns — is typical of a partially blocked flue, a pressure switch with a deteriorating diaphragm, or a fan that is struggling but not yet fully failed. The boiler's 12-minute retry cycle can sometimes allow conditions to recover temporarily. Do not be reassured by the boiler appearing to run normally between lockouts; the underlying fault is still present and should be investigated by a Gas Safe engineer before it becomes a more serious problem.