Ferroli A08 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli A08 fault code mean?
The A08 code means your Ferroli boiler has detected overheating at the heat exchanger and has locked itself out as a safety precaution. Ferroli uses 'A' prefix codes to indicate a hard lockout — the boiler will not attempt to restart on its own and must be manually reset once the underlying problem is resolved. The trigger is the overtemperature protection circuit intervening to prevent damage to the heat exchanger, which can happen when the heating sensor has failed, water is not circulating properly around the system, or air is trapped in the circuit. You may also see this fault appear as F08 in its blocking (non-lockout) form before it escalates to A08.
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 overtemperature or heating sensor Common
The sensor responsible for monitoring heat exchanger temperature can fail or drift out of calibration over time. When it sends an incorrect signal — or no signal at all — the boiler's control board interprets this as an overtemperature event and shuts down. Wiring to the sensor should also be checked, as a broken or loose connection produces the same symptoms as a dead sensor.
- Circulation pump failure or seizure Common
If the pump is not moving water around the heating circuit at the correct rate, heat builds up rapidly in the heat exchanger and the overtemperature protection trips. Pumps can seize after a period of inactivity (common after summer), wear out gradually, or fail electrically. A pump running but not delivering adequate flow can be just as problematic as one that has stopped entirely.
- Air locked in the heating circuit Sometimes
Air pockets trapped in radiators or pipework restrict water flow through the heat exchanger. Even partial blockages from air can cause localised overheating sufficient to trigger A08. This is more likely after recent system work, a pressure loss event, or if radiators have not been bled for some time.
- Sludge or scale restricting the heat exchanger Sometimes
Magnetite sludge and limescale deposits accumulate inside the heat exchanger over years of use, narrowing the waterways and reducing heat transfer. Water cannot flow or cool efficiently, so temperatures rise until the safety circuit intervenes. Systems without a magnetic filter or regular servicing are particularly susceptible.
- PCB or wiring fault Rare
Damage to the wiring harness between the temperature sensor and the printed circuit board, or a fault on the PCB itself, can cause the board to misread sensor data and trigger a false overtemperature lockout. This is less common but should be investigated if sensor and pump checks come back clear.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on your Ferroli's control panel — it is usually marked with an 'R' or a flame symbol with a line through it. Press and hold it for one to two seconds. If the boiler fires and runs normally, monitor it closely over the next few hours. If A08 returns within a short period, do not keep resetting; a genuine fault is present and repeated resets risk damaging the heat exchanger.
- Bleed your radiators to remove trapped air DIY safe
Turn the heating off and allow the system to cool. Use a radiator bleed key to open the bleed valve at the top of each radiator until water (not air) comes out steadily, then close the valve. Work from the ground floor upwards. After bleeding, check the system pressure on the boiler gauge — it should read between 1.0 and 1.5 bar when cold. Top up via the filling loop if it has dropped below 1 bar.
- Check your boiler's temperature settings are not set excessively high DIY safe
Review the central heating flow temperature on your boiler's controls. Running a combi or system boiler at maximum output continuously puts extra thermal stress on components. If the dial is turned to maximum, try reducing it slightly and resetting. This will not fix an underlying fault but rules out user settings as a contributing factor.
- Have a Gas Safe engineer inspect and test the overtemperature sensor Gas Safe engineer
An engineer will check the sensor's wiring for continuity, measure its resistance against the manufacturer's specification, and confirm whether it is reading correctly. A faulty sensor or broken wire is one of the most straightforward A08 repairs and is usually resolved by replacing the sensor and any damaged cabling.
- Have the circulation pump assessed and replaced if necessary Gas Safe engineer
The engineer will check whether the pump is running, delivering adequate flow, and not drawing excessive current. A seized pump can sometimes be freed, but a worn or electrically failed pump will need replacing. Pump replacement on a Ferroli is a common repair with widely available compatible parts.
- Consider a power flush if sludge or scale is suspected Gas Safe engineer
If the engineer finds evidence of magnetite contamination — discoloured water, cold spots on radiators, or restricted flow — a power flush may be recommended. This clears debris from the entire heating circuit and is often combined with fitting a magnetic system filter to prevent recurrence. Without addressing sludge, the A08 fault is likely to return.
- Have the PCB and wiring harness checked if other causes are ruled out Gas Safe engineer
If sensor, pump, and sludge checks all come back clear, the engineer will inspect the wiring harness and PCB for signs of corrosion, damage, or component failure. PCB replacement is more involved and costly, but is sometimes necessary when all other components test correctly.
- Call a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
An A08 lockout that keeps returning after a reset, or that cannot be explained by air in the system, always requires a qualified Gas Safe engineer. Do not continue resetting the boiler in the hope it clears — repeated overtemperature events can cause permanent damage to the heat exchanger, which is a much more expensive repair.
Parts you may need
- Overtemperature / heating sensor (NTC thermistor) · from £25
- Circulation pump (Ferroli compatible) · from £85
- PCB (printed circuit board) · from £150
- Magnetic system filter (e.g. Fernox TF1 or Magnaclean) · from £55
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–£350, depending on the underlying cause.
Frequently asked questions
Can I keep resetting my Ferroli A08 to get hot water while I wait for an engineer?
It is best not to. A08 is a safety lockout triggered because the heat exchanger reached an unsafe temperature. Repeatedly forcing the boiler to fire without fixing the root cause — most commonly a failing pump or faulty sensor — risks warping or cracking the heat exchanger. A damaged heat exchanger can cost £500 or more to replace, far exceeding the cost of a sensor or pump repair. One reset to check whether the fault clears is reasonable; beyond that, leave the boiler off and arrange an engineer visit promptly.
What is the difference between Ferroli F08 and A08?
Ferroli uses two fault-code prefixes to indicate fault severity. An 'F' code (F08) is a blocking fault — the boiler pauses but may attempt to restart automatically once the triggering condition passes. An 'A' code (A08) is a full lockout — the boiler shuts down completely and will not restart until someone manually presses the reset button. If you first saw F08 and it has now changed to A08, the condition that caused the initial fault has not resolved itself and requires investigation.
How much does it cost to fix a Ferroli A08 fault in the UK?
For most homeowners, the repair falls between £100 and £350 all-in. A sensor replacement with a call-out is typically £100–£150, while a circulation pump replacement usually costs £150–£300 depending on the specific model and labour time. If the system needs a power flush due to sludge, expect to pay £300–£600 for that work alone, though this is often money well spent to protect the heat exchanger long-term. A PCB replacement — needed only if all other components test fine — generally runs £200–£300. Ferroli parts tend to be reasonably priced compared to some premium brands, though availability can vary on older discontinued models.
How can I prevent the A08 fault from coming back?
Annual servicing by a Gas Safe engineer is the single most effective measure — a service cleans the heat exchanger, checks the pump and sensor operation, and catches developing problems before they trigger a lockout. If your system does not already have a magnetic filter fitted, ask your engineer to install one: it captures the iron oxide sludge that gradually restricts heat exchanger flow. Bleeding radiators once a year and keeping system pressure between 1.0 and 1.5 bar also help maintain healthy circulation and reduce the risk of A08 returning.