Ferroli F09 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli F09 fault code mean?
The F09 code on a Ferroli boiler signals that the overtemperature protection circuit has been triggered in connection with the return temperature sensor. In plain terms, the boiler has either lost confidence in the reading from the sensor that monitors water returning from your heating circuit, or it has detected that the returning water is dangerously hot — most often because circulation has broken down or air has become trapped in the system. To protect itself from heat damage, the boiler shuts down. Ferroli uses two related code formats: F09 is a blocking fault, meaning the boiler may recover automatically once the underlying issue is resolved, whereas A09 is a lockout variant that requires a manual reset before the boiler will attempt to fire again. Both codes share the same root causes and diagnostic approach.
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
- Trapped air in the heating system Common
Air pockets in radiators or pipework disrupt the flow of water around the circuit. When circulation is impaired, the boiler cannot distribute heat effectively and the return water temperature reading becomes erratic or too high, triggering the overtemperature protection. This is one of the most frequent causes of F09, particularly after a system refill or if radiators have not been bled for some time.
- Faulty or damaged return temperature sensor Common
The return thermistor (temperature sensor) can fail over time due to corrosion, a broken wire, or an internal short circuit. If the sensor sends an implausible signal to the PCB — or no signal at all — the boiler interprets this as an overtemperature condition and shuts down. You may notice the fault appearing even when the system seems to be circulating normally, which points strongly to the sensor itself.
- Poor or failed water circulation (pump issue) Sometimes
If the circulating pump has seized, is running at reduced speed, or radiator valves have been left closed, water cannot move around the system at the correct rate. The boiler then heats the same volume of water repeatedly, causing temperatures to rise sharply at the return. A pump fault often produces F09 alongside other symptoms such as radiators that are cold at the bottom or a boiler that gets very hot very quickly.
- Sludge or blockage restricting flow Sometimes
Over years of use, magnetite sludge and scale can build up inside the heat exchanger, pipework, or pump, narrowing the waterways and reducing flow rate. Even without a complete pump failure, restricted flow can cause localised overheating that triggers the return sensor protection circuit.
How to fix it
- Attempt a single boiler reset DIY safe
Locate the reset button on the front panel of your Ferroli boiler. Press and hold it for approximately 10 seconds. The boiler should attempt to restart and, if the underlying condition has cleared, the fault code will disappear. Avoid pressing reset more than two or three times in succession — repeated resets without resolving the root cause can cause additional issues and will not fix the fault.
- Bleed your radiators to remove trapped air DIY safe
Switch the heating off and allow the system to cool slightly. Starting with the radiator furthest from the boiler and working back towards it, use a radiator bleed key to open the bleed valve (usually found at the top corner of each radiator) a half-turn. Hold a cloth underneath to catch any drips. When water flows steadily rather than air hissing out, close the valve. Once all radiators are bled, check the boiler pressure gauge — if it has dropped below 1.0 bar, top up the system using the filling loop until it reads between 1.0 and 1.5 bar, then reset the boiler again.
- Check that all radiator valves are fully open DIY safe
Walk around your home and confirm that every thermostatic radiator valve (TRV) and lockshield valve is open. A zone where multiple valves are closed can starve the system of flow, sending temperatures too high on the return. Ensure the boiler's own isolating valves are also fully open.
- Check that your gas supply is active DIY safe
Confirm that other gas appliances in your home — such as a gas hob — are working normally. If there is no gas supply to the property, contact your gas supplier. Do not attempt to inspect or adjust gas pipework yourself.
- Have the return temperature sensor tested and replaced if necessary Gas Safe engineer
A Gas Safe registered engineer will use a multimeter to check the resistance of the return thermistor against its expected temperature-resistance curve. A sensor reading outside its normal range needs to be replaced. This is a straightforward job for an engineer but requires accessing internal components — do not remove the boiler casing yourself.
- Have the circulating pump inspected Gas Safe engineer
If bleeding the radiators and checking the valves does not resolve the fault, a Gas Safe engineer should check whether the pump is running at the correct speed and pressure. A seized or underperforming pump will need to be freed or replaced. The engineer may also carry out a system power-flush if sludge build-up is suspected.
- Call a Gas Safe registered engineer if the fault persists or keeps returning Gas Safe engineer
If the F09 or A09 code comes back after resetting, or if you have already bled the radiators and checked the valves without success, contact a Gas Safe registered engineer. They can carry out a full diagnostic, check wiring to the sensor and PCB, assess pump performance, and make any necessary repairs safely. Do not continue resetting a boiler that repeatedly locks out — this can mask a worsening fault.
Parts you may need
- Return temperature sensor (thermistor) · from £25
- Circulating pump · from £85
- Pump head (motor only) · from £45
- System inhibitor/sludge remover (e.g. Fernox F3) · from £18
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–£280, depending on the underlying cause.
Frequently asked questions
What is the difference between F09 and A09 on a Ferroli boiler?
Both codes relate to the same overtemperature protection fault linked to the return sensor, but they behave slightly differently. An F09 is a blocking fault — the boiler shuts down but may reset itself automatically if the triggering condition clears (for example, if trapped air disperses). An A09 is a lockout fault, meaning the boiler will not restart until you manually press the reset button. If your display shows A09, always reset once and monitor whether the fault returns.
Can I fix the Ferroli F09 fault myself?
Some of the preliminary checks are perfectly safe for a homeowner: bleeding radiators, checking that all valves are open, topping up system pressure, and performing one or two resets. However, if these steps do not clear the fault, the cause is likely a failed return sensor, a pump fault, or an internal wiring issue — all of which must be diagnosed and repaired by a Gas Safe registered engineer. Never remove the boiler casing or touch any internal components yourself.
How much does it cost to fix a Ferroli F09 fault in the UK?
For most people, the repair falls between £90 and £280. A straightforward sensor replacement typically costs £80–£130 including parts and labour, while a pump replacement usually runs £150–£280 all in. If the engineer's visit reveals that a power-flush is needed to clear sludge, this adds to the cost — a full system flush on a typical UK home is usually £300–£500 as a separate job. In rare cases where the PCB has been damaged by prolonged overtemperature events, control board replacement can push costs higher, but this is uncommon as a primary cause of F09.
Why does my Ferroli keep showing F09 even after I reset it?
A fault that returns quickly after reset almost always means the underlying problem has not been resolved. The three most likely culprits are a failing return temperature sensor that continuously sends bad data to the PCB, a pump that is not circulating water at the correct rate, or a heavily sludged system restricting flow. Repeated resets without fixing the root cause will not help and can cause additional wear. Book a Gas Safe engineer to carry out a proper diagnosis rather than continuing to reset.