Ferroli A41 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli A41 fault code mean?
The A41 code is a manual lockout fault on Ferroli boilers — meaning the boiler shuts itself down and will not restart without you pressing the reset button first. It points to a problem with the flow (CH) temperature sensor, also known as the NTC thermistor. This is the component responsible for monitoring how hot the water leaving the boiler is. When the boiler cannot get a reliable reading from this sensor — whether because the sensor has physically shifted away from the flow pipe, its wiring has come loose, or the sensor itself has failed — it triggers A41 and locks out as a safety precaution. In essence, the boiler has lost confidence in its own temperature data and refuses to fire until the situation is resolved.
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
- Flow sensor physically dislodged from the pipe Common
The NTC thermistor is clipped or strapped against the flow pipe to measure outgoing water temperature. Vibration, a previous service, or simply age can cause it to slip out of full contact with the pipe. When this happens, the sensor no longer accurately reads water temperature, triggering the A41 lockout. This is the most commonly cited cause in Ferroli's own documentation for this fault.
- Faulty or failed NTC thermistor Common
Like all electronic components, the flow thermistor degrades over time. Internal resistance can shift outside of the acceptable range, or the sensor can fail outright — particularly on older Ferroli units. An engineer can confirm this quickly using a multimeter and comparing the resistance reading against Ferroli's specification table. A failed sensor requires replacement.
- Wiring fault between sensor and PCB Sometimes
The wires connecting the flow sensor to the control board can suffer from corrosion, heat damage, or a loose connector over years of operation. A break or high-resistance connection in this wiring produces the same effect as a failed sensor — the PCB receives no usable signal and locks the boiler out on A41.
- PCB fault misreading the sensor signal Rare
In less common cases, the control board itself may be misinterpreting a perfectly good sensor signal due to a fault on the board. This is typically only considered once the sensor and wiring have been confirmed as good, and it represents a more significant repair.
How to fix it
- Press the reset button to attempt to clear the lockout DIY safe
Locate the reset button on your Ferroli boiler — it is usually marked with an 'R' symbol or a flame icon. Press and hold it for approximately 5 seconds until the boiler attempts to restart. If the boiler fires up and the A41 code does not return within a short period, the fault may have been a temporary one-off. Do not reset the boiler more than two or three times in total — repeated resets without fixing the underlying cause can damage the heat exchanger.
- Check that the fault code does not immediately return DIY safe
After resetting, keep an eye on the boiler display for the next 15–30 minutes. If A41 reappears quickly or the boiler fails to restart at all, there is a genuine fault with the sensor or its wiring that needs a qualified engineer to diagnose. Do not continue resetting in the hope it will eventually clear.
- Contact a Gas Safe registered engineer for diagnosis Gas Safe engineer
A Gas Safe engineer will physically inspect the flow sensor to check whether it has simply moved out of position on the pipe — the official Ferroli guidance specifically advises checking the sensor's positioning and contact. If repositioning resolves it, the fix is straightforward. If not, they will test the thermistor's resistance with a multimeter to determine whether the sensor itself has failed and needs replacing.
- Engineer to test and replace the flow NTC thermistor if required Gas Safe engineer
If the sensor is confirmed faulty, the engineer will fit a replacement NTC thermistor suitable for your Ferroli model. Ferroli parts are generally competitively priced, though lead times can be longer on older or discontinued models. Once replaced, the engineer will reset the boiler, run it through a heating cycle, and confirm the fault has cleared and temperature readings are within normal range.
- Engineer to inspect wiring and PCB if sensor replacement does not resolve the fault Gas Safe engineer
Should A41 persist after a new sensor is fitted, the engineer will trace the wiring loom from the sensor to the PCB looking for corrosion, chafing, or a failed connector. If the wiring is sound, attention turns to the control board itself, which may need replacing — a more costly repair that is worth weighing against the age of the boiler.
Parts you may need
- Ferroli flow NTC thermistor (temperature sensor) · from £25
- Sensor wiring harness / connector · from £15
- Ferroli PCB (control board) · from £120
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 £90–£200, depending on the underlying cause.
Frequently asked questions
Can I fix the Ferroli A41 fault code myself?
The only safe DIY step is pressing the reset button — once or twice at most. Everything else, including checking the sensor's position, testing it with a multimeter, or replacing it, involves accessing the internals of the boiler and should only be done by a Gas Safe registered engineer. Attempting internal repairs yourself is unsafe and will invalidate any remaining warranty.
How much does it cost to fix an A41 fault on a Ferroli boiler?
For most people, the repair involves either repositioning the existing sensor (low cost — mainly a call-out and labour fee of roughly £90–£120) or replacing the NTC thermistor (typically £90–£200 including parts and labour). If the fault turns out to be a wiring issue, costs are similar. A PCB replacement is a more expensive outlier — typically £250–£350 all in — but this is rarely the cause of an A41 code and would only be considered after the sensor and wiring have been ruled out.
Why does the A41 code keep coming back after I reset it?
If A41 returns repeatedly after resetting, it means the underlying hardware fault has not been resolved. Resetting only clears the lockout temporarily — it does not repair a displaced, damaged, or wiring-faulted sensor. Continuing to reset without fixing the cause risks damaging other components such as the heat exchanger. Book an engineer to diagnose and repair the root cause.
Is A41 the same as an F10 fault on a Ferroli boiler?
They are related but not identical. F10 on a Ferroli boiler also indicates a flow sensor issue — typically a short circuit or open circuit on the NTC thermistor — and is a blocking fault that may clear automatically once conditions improve. A41 is a harder lockout that always requires a manual reset. Both ultimately point to the same component (the flow temperature sensor), so the diagnosis and repair process is very similar.