Ferroli A23 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli A23 fault code mean?
The Ferroli A23 code has two distinct meanings depending on which model you own. On the DOMIcondens HE 26C and Optimax HE Plus, A23 indicates that the system water pressure failed to reach its required level within approximately four minutes — the boiler locks out as a safety measure. On the Modena and certain other Ferroli models, A23 points to a card or PCB parameter error, meaning the control board has detected an incorrect or corrupted configuration setting. Check your model before proceeding, as the diagnosis and fix differ significantly between these two interpretations.
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
- Low system water pressure (pressure variant models) Common
The most frequent trigger on DOMIcondens HE 26C and Optimax HE Plus units. If system pressure drops below roughly 1 bar, the boiler cannot complete its start sequence and displays A23. Pressure naturally drops a little over time, but a sudden drop usually signals a leak or air in the system.
- Water leak on radiator valves or towel rail valves Common
Small leaks at radiator valves, lockshields, or towel rail connections are responsible for the majority of pressure-loss faults on Ferroli systems. The leak may be slow and only visible as a slight weeping or dried limescale residue around a fitting. Check all visible joints and valve bodies carefully.
- Air trapped in the radiators Common
If no leak is present, trapped air is the next most likely culprit. Air pockets prevent the system from maintaining correct pressure and can cause the boiler to misread the pressure sensor. Bleeding the radiators often resolves this.
- Faulty or weeping pressure relief valve (PRV) Sometimes
A PRV that has partially lifted and not reseated properly will continuously release water, causing a slow but persistent pressure drop. Look for signs of water discharge from the overflow pipe outside the property.
- Failing expansion vessel Sometimes
If the expansion vessel has lost its pre-charge pressure or the internal membrane has failed, the system cannot absorb pressure fluctuations correctly and pressure may drop below the A23 threshold repeatedly. This requires an engineer to inspect and re-pressurise or replace the vessel.
- PCB or card parameter error (Modena and other models) Sometimes
On affected models, the control board reads an incorrect parameter value — possibly caused by a firmware corruption, a loose ribbon cable, or a board that has been replaced with one carrying the wrong configuration. The boiler cannot operate safely with an invalid parameter set and locks out.
- Faulty water pressure sensor Rare
If the pressure sensor itself is sending an inaccurate reading, the boiler may display A23 even when actual system pressure is perfectly acceptable. An engineer can test sensor output with a multimeter to confirm or rule this out.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button — on most Ferroli models it is marked with an 'R' or a flame symbol on the fascia. Press and hold it for five to ten seconds. If the boiler fires up and runs normally, note whether the fault returns. A one-off reset after a temporary pressure dip is reasonable, but do not reset more than two or three times in quick succession if the code keeps reappearing.
- Check the pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. For normal operation it should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1 bar, low pressure is almost certainly the cause of the A23 on pressure-variant models.
- Top up system pressure via the filling loop DIY safe
Locate the filling loop — usually a short silver braided hose with one or two valves fitted beneath the boiler. Open the valve(s) slowly and watch the pressure gauge rise. Stop when it reaches 1.2–1.5 bar, then close the valve(s) firmly. Reset the boiler and check the pressure again after an hour. If pressure drops again within a day or two, there is a leak or expansion vessel problem that needs professional attention.
- Inspect visible pipework, valves, and radiator connections for leaks DIY safe
Walk around every radiator and towel rail and look closely at valve bodies, unions, and bleed points for signs of moisture, rust staining, or dried limescale deposits. Check under the boiler casing for any drips. If you find a weeping valve, try tightening the gland nut very slightly — but do not force it. Any active leak beyond a simple gland tightening should be referred to a plumber or Gas Safe engineer.
- Bleed the radiators to remove trapped air DIY safe
If pressure appeared normal or returns to normal after topping up but the fault recurs, bleeding the radiators can help. Use a radiator bleed key on the bleed valve at the top corner of each radiator. Hold a cloth underneath and open the valve slowly until water (not air) starts to flow, then close it. Work from the ground floor upwards. After bleeding, recheck system pressure and top up if needed.
- Have an engineer inspect the expansion vessel Gas Safe engineer
If pressure continues to drop despite no visible leaks and bled radiators, the expansion vessel is likely the culprit. A Gas Safe engineer will check the pre-charge pressure using a tyre-style gauge on the Schrader valve and re-pressurise or replace the vessel as needed. This is not a DIY task.
- Have an engineer investigate the pressure sensor Gas Safe engineer
If actual pressure is confirmed correct but the A23 fault persists, the water pressure sensor may be faulty. An engineer will test it electrically and replace it if it is giving erroneous readings.
- Have an engineer diagnose and resolve a PCB parameter error (Modena and similar models) Gas Safe engineer
On models where A23 signals a card parameter error, the engineer will check all wiring connections to the control board, look for signs of corrosion or heat damage, and use diagnostic tools to verify the parameter configuration. A loose or damaged connector is sometimes all that needs attention. If the board itself is corrupted or failed, it will need to be repaired or replaced — a job strictly for a qualified professional.
- Call a Gas Safe registered engineer if the fault persists or keeps returning Gas Safe engineer
If you have topped up the pressure, bled the radiators, and reset the boiler but A23 continues to appear — or returns within a short time — stop resetting and call a Gas Safe registered engineer. Repeated lockouts indicate an underlying fault that will not resolve itself and may worsen if ignored.
Parts you may need
- Water pressure sensor · from £35
- Expansion vessel (replacement) · from £60
- Pressure relief valve · from £25
- Ferroli PCB / control board · from £180
- Filling loop assembly · from £20
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 fix the Ferroli A23 fault myself?
If the cause is low system pressure, yes — topping up the system through the filling loop and bleeding the radiators are safe homeowner tasks. However, if pressure keeps dropping, if there is an active leak beyond a simple gland tightening, or if your model is showing the PCB parameter variant of A23, you will need a Gas Safe registered engineer. Never attempt to work on internal boiler components, the gas valve, or the control board yourself.
Why does my Ferroli boiler keep losing pressure and showing A23?
If you top up the pressure and the gauge drops again within a day or two, you almost certainly have a leak somewhere in the system — even if it is a small one. Common culprits are radiator valve glands, towel rail unions, and the boiler's own pressure relief valve. A failing expansion vessel can also cause repeated pressure loss without an obvious leak. An engineer can locate the source and fix it permanently; simply topping up the pressure is only a temporary measure.
How much does it cost to fix the Ferroli A23 fault in the UK?
For pressure-related causes, most homeowners pay between £100 and £200 for a call-out, diagnosis, and a straightforward fix such as replacing a pressure sensor or re-pressurising an expansion vessel. Replacing the expansion vessel itself typically adds £60–£100 in parts. If A23 is caused by a PCB parameter error, a control board replacement on a Ferroli can cost £280–£400 including parts and labour — and on an older boiler out of warranty, it is worth getting a replacement boiler quote at the same time for comparison.
What is the difference between Ferroli fault codes A23 and A37?
On pressure-variant Ferroli models, both A23 and A37 are related to system water pressure, but they trigger under slightly different conditions. A23 typically means the pressure failed to reach the required level during start-up, while A37 usually indicates pressure has dropped below an acceptable threshold during normal operation. The diagnostic approach is the same for both: check for leaks, bleed radiators, top up pressure, and have an engineer inspect the expansion vessel and pressure sensor if the fault recurs.