Ferroli A35 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli A35 fault code mean?
The A35 code appears on Ferroli boilers — including the Optimax HE Plus 31C and 38C and the DOMIcondens HE 26C — when the boiler's control board detects that the incoming mains electricity is operating at an abnormal or unstable frequency. The UK standard is 50 Hz; if the supply deviates significantly from this, the boiler locks out as a precaution. This is an electrical fault rather than a gas or heating-circuit issue, and it sits alongside related codes such as A34, A36, and A62 in Ferroli's electrical fault group.
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
- Unstable or irregular mains electricity supply Common
A brief power cut, grid fluctuation, or voltage spike can cause the mains frequency to stray from 50 Hz. The control board flags this immediately and locks the boiler out as a precaution. If the disturbance was temporary, a single reset is often enough to get the boiler running again.
- Generator or non-standard power source in use Sometimes
Running the boiler from a portable generator, an uninterruptible power supply (UPS), or any backup power unit that does not produce a clean, stable 50 Hz sine wave will reliably trigger A35. The boiler expects grid-quality power and will refuse to operate on a sub-standard supply.
- Faulty PCB (printed circuit board) Sometimes
If the mains supply is confirmed stable but the fault persists, the control board itself may be misreading the frequency signal due to a component failure or a break in the internal circuit. A Gas Safe engineer will test the board with a multimeter to identify any breaks before recommending a replacement.
- Damaged or degraded electrical wiring Rare
Loose connections, corroded terminals, or damaged wiring between the mains input and the PCB can introduce signal noise that mimics a frequency anomaly. This is less common but worth checking during a full electrical inspection.
How to fix it
- Check that the boiler is running on a normal mains supply DIY safe
Make sure the boiler is plugged into — or wired to — your standard household mains circuit, not a generator or UPS. If there has been a recent power cut or fluctuation in your area, this is most likely the cause. Wait a few minutes to allow the supply to fully stabilise before attempting a reset.
- Reset the boiler once or twice DIY safe
Locate the reset button (often marked with an 'R' or a flame symbol) and hold it for 5–10 seconds as described in your boiler manual. If the boiler restarts and runs normally, the fault was probably triggered by a one-off supply issue. Do not reset more than two or three times in quick succession — repeated resets when an underlying fault exists can damage the heat exchanger or fan.
- Monitor whether the fault returns DIY safe
Keep an eye on the boiler for the next hour or two. If the A35 code reappears within a short period, the cause is something beyond a temporary supply glitch and the boiler needs a professional inspection. Continuing to reset it will not resolve the underlying problem.
- Arrange an inspection by a Gas Safe registered engineer Gas Safe engineer
A qualified engineer will use a multimeter to test the mains input, check wiring integrity throughout the electrical circuit, and assess the PCB. If the wiring is sound, the PCB may need replacing. In some cases a faulty timer or thermostat control may be contributing, and the engineer will check these components too. Do not attempt to open the boiler casing or handle any electrical components yourself.
Parts you may need
- Ferroli PCB (control board) · from £120
- Boiler electrical wiring loom · from £45
- Mechanical timer/programmer unit · from £35
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–£320, depending on the underlying cause.
Frequently asked questions
Can I fix the Ferroli A35 fault myself?
The only safe DIY steps are confirming you are on a stable mains supply and performing one or two resets. Everything beyond that — testing wiring, inspecting the PCB, or replacing components — must be carried out by a Gas Safe registered engineer. Boiler electrics require the right diagnostic tools and training to work on safely.
Will a new PCB definitely fix the A35 code?
Not always. The PCB is a possible cause, but an unstable mains supply or damaged wiring can produce exactly the same fault. A good engineer will rule out supply and wiring issues first before recommending a PCB replacement, which typically costs £200–£300 including parts and labour. Replacing the board without diagnosing the root cause risks the new board developing the same fault.
My boiler is over 10 years old and showing A35 — is it worth repairing?
If the fix is straightforward — such as rewiring a loose connection — it is almost certainly worth repairing. However, if the PCB needs replacing and the boiler is already ageing, it is worth getting a boiler replacement quote alongside the repair quote. As a rough rule of thumb, if the repair cost approaches 30–40% of the price of a new boiler, replacement often makes more financial sense in the long run, especially given that parts for older Ferroli models can sometimes be difficult to source.
Could a power cut have caused the A35 fault?
Yes — a power cut, voltage spike, or grid disturbance is one of the most common triggers for A35. When the mains supply drops out and then returns, the frequency can momentarily be outside the 50 Hz tolerance the boiler expects. In this scenario a single reset after the supply has fully stabilised will usually clear the code with no further action needed.