Ferroli A03 Fault Code
The A03 fault is the most homeowner-accessible Ferroli fault code — system pressure has dropped below the minimum operating threshold. In most cases, repressurising the system to 1.2–1.5 bar will clear it without an engineer visit.
What is the Ferroli A03 fault code?
The Ferroli A03 fault is triggered when the pressure sensor detects the sealed heating circuit has fallen below the minimum operating threshold — typically 0.5 bar. The boiler locks out to prevent running with insufficient water, which would cause overheating and damage. A03 is the Ferroli equivalent of Vaillant F.22 and Worcester Bosch F1. Unlike most other Ferroli fault codes, a homeowner can safely address A03 by repressurising the system — provided there is no visible leak.
What causes the A03 fault?
All sealed heating systems lose a small amount of pressure over time through micro-permeation. A drop of 0.1–0.2 bar per year is normal.
A dripping radiator valve, weeping joint, or leaking pump gland causes pressure to drop more rapidly — weekly or monthly rather than annually.
Bleeding radiators releases water alongside air. Always top up the system pressure after any bleed.
If the expansion vessel loses its air charge, pressure fluctuates dramatically and can drop below the A03 threshold overnight even when no water has escaped.
A stuck-open automatic air vent slowly allows water to escape from the system, causing gradual pressure loss.
If both filling loop valves are not completely closed after repressurising, the internal seal can allow slow drainage.
What you can safely check yourself
Cold system pressure should be 1.0–1.5 bar. Below 0.8 bar, repressurise. If the gauge reads zero or near zero, check for visible leaks before adding water.
Open the filling loop valves under the boiler slowly until the gauge reads 1.2–1.5 bar, then close both valves fully. On Ferroli models the filling loop is usually a flexible braided hose with two quarter-turn valves.
Press the reset button to clear A03. If pressure drops again within a week, there is likely a leak or a failed expansion vessel — call a Gas Safe engineer.
Do not reset the boiler more than once if the fault code returns immediately. Repeated resets without diagnosis can mask a worsening fault.
What needs a Gas Safe engineer
- Pressure test the system to locate any hidden leak
- Inspect and test the expansion vessel — recharge with nitrogen or replace
- Check automatic air vents for water loss
- Inspect pump gland seals for weeping
- Check radiator valve gland nuts for drips
- Replace filling loop or internal valves if internal leakage confirmed
- Fit a magnetic filter if not already present
Expansion vessel failures are common across South London. Thames Water's hard water accelerates corrosion of the rubber diaphragm inside the expansion vessel. Across Wandsworth, Lewisham, Lambeth, Merton, Southwark, Greenwich, and Bromley, Ferroli boilers on 10+ year old installations frequently have completely flat expansion vessels. Reion checks expansion vessel pressure on every annual service call.
How much does a A03 repair cost?
Costs below are UK averages. Reion provides a fixed quote after the £80 diagnostic visit — no obligation to proceed.
How to reset a Ferroli boiler showing A03
Check the pressure gauge — repressurise to 1.2–1.5 bar if below 1 bar.
Ensure both filling loop valves are fully closed after repressurising.
Press the reset button to clear the A03 lockout.
Monitor the pressure gauge over the next week.
If pressure drops more than 0.3 bar within a few days, call a Gas Safe engineer.
A03 alone is rarely a reason to consider replacing a Ferroli boiler. The exception is if the source of the pressure loss is a cracked heat exchanger — in which case the repair cost on an older unit can make replacement the better financial decision.
Common questions about the Ferroli A03 fault
Can I repressurise a Ferroli boiler myself?
Why does my Ferroli boiler keep showing A03?
What pressure should a Ferroli boiler be set to?
Is the Ferroli A03 fault dangerous?
How do I know if my Ferroli needs a new expansion vessel?
Other Ferroli fault codes
Need a Ferroli engineer in London?
Reion is Gas Safe registered (919881), based in South London, and available 24/7. Fixed price after the diagnostic visit — no hidden charges.
