Ferroli A43 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli A43 fault code mean?
The A43 fault code indicates that your Ferroli boiler has triggered its heat exchanger protection function. In plain terms, the boiler has detected insufficient water circulation through the heat exchanger and has shut itself down to prevent overheating damage. Because A43 begins with the letter 'A', it is a lockout fault — the boiler will not restart on its own and requires a manual reset once the underlying cause has been identified and resolved. This distinguishes it from the closely related F43 code seen on other Ferroli models (such as the DOMIproject range), where the fault may clear automatically once conditions improve. The A43 is most commonly associated with the Ferroli DOMIcondens HE 26 C, but the same root cause applies across models that display either code.
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
- Circulation pump failure or seizure Common
The most likely culprit. If the pump has seized — common after a period of inactivity such as at the end of summer — water cannot flow through the heat exchanger quickly enough. The boiler detects the resulting temperature spike and trips out on A43. A seized pump may sometimes be freed by an engineer, though replacement is often the more reliable fix.
- Air trapped in the heating system Common
Air locks in the pipework or radiators reduce effective water flow. Even a partial air lock can restrict circulation enough to trigger exchanger protection, particularly during or after a cold snap when the boiler is working harder than usual.
- Sludge or magnetite build-up restricting flow Common
In older heating systems, black iron-oxide sludge (magnetite) accumulates in the pipework, radiators, and heat exchanger itself. This narrows the waterways, reduces flow rate, and causes localised hot spots that the boiler interprets as a circulation failure.
- Low system pressure Sometimes
If system pressure has dropped below around 0.8 bar, there may not be enough water in the circuit to circulate adequately. This is worth checking before anything else as it takes seconds to confirm on the pressure gauge.
- Faulty heat exchanger temperature sensor Sometimes
The sensor that monitors heat exchanger temperature may itself be giving inaccurate readings, causing the boiler to believe circulation has failed even when flow is normal. This is less common than a genuine pump or flow issue but should be tested if the system checks out mechanically.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler fascia. It should read between 1.0 and 1.5 bar when cold. If it is below 1.0 bar, top up via the filling loop until the needle sits around 1.2 bar, then close the filling loop valves fully. A boiler low on water cannot circulate properly.
- Bleed your radiators to remove trapped air DIY safe
Turn the heating on, let it run for a few minutes, then turn it off and let the system cool slightly. Use a radiator bleed key to open each radiator bleed valve in turn — starting upstairs and working down. Release air until water dribbles out, then close the valve. Re-check system pressure afterwards and top up again if needed.
- Reset the boiler once DIY safe
With the above checks done, press and hold the reset button (usually marked with an 'R' or a flame symbol) for around five seconds. Allow the boiler a full minute to attempt ignition and return to normal operation. If it fires up successfully and stays running, monitor it over the next 24 hours. Do not reset more than two or three times in total — repeated resets on a genuine circulation fault risk damaging the heat exchanger.
- Check that all lockshield and thermostatic radiator valves are open DIY safe
A common oversight is finding that several TRVs or lockshield valves have been closed right down, severely restricting flow. Make sure at least a couple of radiators have fully open valves to give the pump a clear circuit to push water around.
- Have a Gas Safe engineer inspect the circulation pump Gas Safe engineer
If the fault returns after a reset, the pump is the most likely cause. An engineer will check whether the pump is seized, running at the correct speed, and moving enough water. A pump that has been seized may sometimes be freed mechanically; otherwise it will need replacing. This work must be carried out by a Gas Safe registered engineer.
- Arrange a powerflush if sludge is suspected Gas Safe engineer
If the system is several years old, radiators take a long time to heat up, or there is evidence of black water when bleeding radiators, a powerflush may be necessary. This process forces a cleaning agent through the entire system to dislodge and flush out magnetite and debris. A Gas Safe or qualified heating engineer will carry this out, and it is worth asking about fitting a magnetic filter at the same time to prevent sludge returning.
- Have the heat exchanger temperature sensor tested or replaced Gas Safe engineer
If pump and flow checks come back normal, a Gas Safe engineer should test the NTC sensor on the heat exchanger. A faulty sensor can produce false A43 faults even when circulation is fine. Sensor replacement is a straightforward job for an engineer and is relatively inexpensive.
- Call a Gas Safe registered engineer if the fault persists or keeps returning Gas Safe engineer
If A43 comes back after basic checks, or you are not confident carrying out the DIY steps above, book a visit from a Gas Safe registered engineer. They have the diagnostic tools to identify whether the problem is the pump, sludge, a sensor, or a less common underlying fault. You can verify any engineer's Gas Safe registration at gassaferegister.co.uk.
Parts you may need
- Circulation pump (compatible replacement) · from £65
- Heat exchanger NTC temperature sensor · from £25
- Magnetic system filter (e.g. Fernox TF1 or equivalent) · from £55
- Radiator bleed key · from £3
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 £120–£350, depending on the underlying cause.
Frequently asked questions
Will the Ferroli A43 fault clear on its own?
No. Unlike an F43 blocking fault (which can reset itself once conditions return to normal), the A43 is a lockout code — the boiler will stay off until you manually press the reset button. Simply waiting will not resolve it. You need to find and fix the underlying cause first, then reset.
What is the difference between Ferroli A43 and F43?
Both codes point to the same problem: the boiler's heat exchanger protection has activated due to poor circulation. The difference is in how the boiler responds. An 'F' code is a blocking fault that may clear automatically once flow improves. An 'A' code is a full lockout — the boiler stays off and requires a manual reset. The A43 tends to appear on the DOMIcondens HE range, while F43 is more common on DOMIproject and DOMIcondens F models.
How much does it cost to fix a Ferroli A43 fault in the UK?
For most households, the repair falls between £120 and £350. Bleeding radiators and topping up pressure is free to do yourself, or around £80–£150 if you call an engineer out for the visit. A pump inspection and replacement typically costs £150–£250 including labour. A sensor replacement is usually £80–£120 all in. If sludge is the root cause and a powerflush is needed, expect to pay £300–£500 or more depending on system size. On an older Ferroli boiler, if repair costs are approaching 30–40% of a new boiler price, it is worth getting a replacement quote at the same time.
Can I prevent the A43 fault from coming back?
Yes — the most effective long-term measure is fitting a magnetic system filter (such as a Fernox TF1 or similar) if you do not already have one. This captures magnetite sludge before it can accumulate in your pump and heat exchanger. If the system is already heavily sludged, a powerflush should come first. Keeping system pressure between 1.0 and 1.5 bar and bleeding radiators annually also helps maintain healthy circulation.