Ariston 601 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 601 fault code mean?
Ariston fault code 601 sits within the 600-series, which covers flue, fan, and air pressure switch issues — all safety-critical systems that ensure combustion gases leave your home safely. The code has two main interpretations depending on your specific Ariston model. On open-flue (atmospheric) models, it signals that the flue gas overheat sensor has tripped, meaning exhaust gases have reached a temperature beyond safe limits. On room-sealed (fan-assisted) models, the same code can indicate a fan speed or tachometer fault, where the boiler's PCB has detected that the fan is not spinning at the correct speed — or not spinning at all. In either case, the boiler will lock out and refuse to fire until the underlying problem is resolved. Do not continue using the boiler until a Gas Safe registered engineer has assessed it, as both fault types carry a risk of harmful gases not being properly expelled.
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 common trigger on open-flue models. A partial or full blockage — caused by bird or insect nests, debris, or a damaged flue terminal — prevents exhaust gases from escaping freely. This causes flue gas temperatures to build up and trip the overheat sensor. Warmer months see a spike in nest-related blockages.
- Faulty or seized boiler fan Common
On room-sealed models, the fan motor may have burnt-out windings or deteriorated bearings, preventing it from reaching the required speed. Because the fan is responsible for pushing combustion gases out through the flue, the boiler locks out immediately if the PCB cannot confirm the fan is running correctly.
- Flue gas temperature sensor failure Sometimes
The sensor itself can develop an open-circuit fault or internal breakdown, sending an incorrect signal to the PCB even when flue temperatures are normal. This is effectively a false alarm from a failing component rather than a genuine overheating event.
- Poorly fitted or damaged flue system Sometimes
A flue that was incorrectly installed — wrong diameter, insufficient fall, or inadequate joints — can restrict gas flow over time, leading to repeated overheating events. Visible staining or sooting around flue joints is a tell-tale sign.
- Fan wiring or electrical fault Rare
Damaged wiring, a corroded connector, or a failed tachometer (speed-sensing) component in the fan circuit can cause the PCB to read zero or erratic fan speed, triggering a lockout even if the fan motor itself is mechanically sound.
How to fix it
- Check for obvious external flue obstructions DIY safe
If you can safely see the flue terminal on an external wall or roof from ground level, look for any visible blockage such as a bird nest, leaves, or debris covering the outlet. Do not attempt to reach or clear the terminal yourself — simply note what you can see and report it to the engineer.
- Attempt a single boiler reset DIY safe
Press and hold the reset button for approximately five seconds, then wait while the boiler attempts to restart. One reset attempt is reasonable. If fault code 601 returns immediately or after a short time, stop resetting — repeated resets will not fix the underlying fault and can mask useful diagnostic information for the engineer.
- Note any additional symptoms before calling DIY safe
Take note of whether you can hear the fan running (a steady hum or whirring from inside the boiler casing), whether there are any unusual smells near the boiler or flue, or whether the boiler has been cutting out repeatedly over recent days. This information will help the engineer diagnose the fault more quickly.
- Contact a Gas Safe registered engineer for diagnosis and repair Gas Safe engineer
All work beyond the checks above must be carried out by a Gas Safe registered engineer. Depending on the model and fault version, the engineer will inspect and clear or replace the flue, test and replace the fan motor or its wiring, or substitute the flue gas temperature sensor. Working on gas appliances without Gas Safe registration is illegal and dangerous.
Parts you may need
- Boiler flue fan / circulation fan · from £120
- Flue gas temperature sensor (smoke thermostat) · from £35
- Fan wiring harness / connector · from £25
- Standard horizontal flue kit · from £90
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–£375, depending on the underlying cause.
Frequently asked questions
Is Ariston fault code 601 dangerous — can I keep using the boiler?
You should not continue using the boiler while code 601 is active. Both fault types it can represent — flue gas overheating and fan failure — relate to the safe removal of combustion gases from your home. If exhaust gases cannot escape properly, there is a risk of carbon monoxide buildup. Switch the boiler off and arrange for a Gas Safe engineer to inspect it promptly.
How can I tell whether my boiler has the flue overheat version or the fan fault version of code 601?
The simplest clue is whether you can hear the fan running when the boiler tries to start. If the fan is completely silent or makes an unusual grinding noise, a fan fault is more likely. If the fan sounds normal but the boiler still locks out, the flue overheat sensor or a blockage is the more probable cause. Your engineer will confirm which applies to your specific model.
How much does it cost to fix Ariston fault code 601 in the UK?
Most people with this fault pay between £150 and £375 all in. A straightforward flue clearance is often covered within a standard call-out charge of £100–£150. A fan replacement typically runs to £225–£375 including parts and labour. A sensor swap is generally less expensive at around £100–£200. Note that if a full flue replacement is needed — particularly a vertical flue exiting through the roof — costs can rise to £450–£850 or more depending on access; this is an outlying scenario rather than the typical repair.
Can annual servicing prevent fault code 601?
Yes, regular annual servicing significantly reduces the likelihood of this fault appearing. During a service, an engineer will check the flue for blockages or deterioration, test the fan operation and speed, and inspect the condition of temperature sensors. Catching a partially blocked flue or a fan with worn bearings early is far cheaper than dealing with a full lockout and emergency call-out.