Ferroli boiler error codes
16 codes that Ferroli publishes for its boilers, each with what it means, the likely cause, what you can safely check and when to call a Gas Safe registered engineer. Tap your code:
Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone. Anything else is for a Gas Safe registered engineer. If you smell gas, call the National Gas Emergency Service on 0800 111 999. Codes that need one are marked Gas Safe engineer.
Read this first
Ferroli’s manual for the Optimax HE Plus 18 OV, a heat-only condensing boiler (2009), splits its codes in two. An A code shuts the boiler down until you press RESET for 1 second; an F code is a temporary shutdown that clears itself as soon as the value behind it is back in range. If a fault remains after two resets, Ferroli says to contact its Service Centre. The only fault the manual gives the user a fix for is the burner not lighting (A01): the gas cocks. The rest of its troubleshooting table is for the engineer, and is named here only so you know what the visit is about.
A01 Burner does not light
- What it means
- No burner ignition: the boiler has shut down.
- Likely cause
- No gas; a faulty detection or ignition electrode; a defective gas valve; wrong gas inlet pressure; a blocked siphon (condensate trap).
- What you can safely check
Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.
- Make sure the gas cocks ahead of the boiler and on the meter are open, and that there is gas and electricity.
- Press the RESET button for 1 second.
- When to call a Gas Safe registered engineer
- For repeated shutdowns, contact the Ferroli Service Centre or a Gas Safe registered engineer: the electrodes, gas valve, gas pressure and siphon are theirs to check.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
A02 Flame with burner off
- What it means
- A flame was detected with the burner off.
- Likely cause
- A defective ionisation electrode or its wiring, or the main circuit board.
- What you can safely check
Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.
- Press the RESET button for 1 second.
- When to call a Gas Safe registered engineer
- If it remains after two resets, contact the Ferroli Service Centre or a Gas Safe registered engineer. The electrode wiring and circuit board are for the engineer.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
A03 High limit protection
- What it means
- The high-limit (overheat) protection has operated.
- Likely cause
- The flow sensor not working or out of place; no circulation in the system.
- What you can safely check
Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.
- Press the RESET button for 1 second.
- When to call a Gas Safe registered engineer
- If it remains after two resets, contact the Ferroli Service Centre or a Gas Safe registered engineer. Ferroli’s checks — the flow sensor, the pump, the radiator valves and the automatic bypass — are for the engineer.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
A04 Flue gas fault
- What it means
- A flue gas fault: F07 has happened three times in the last 24 hours.
- Likely cause
- The flue.
- What you can safely check
Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.
- Press the RESET button for 1 second.
- When to call a Gas Safe registered engineer
- If it remains after two resets, contact the Ferroli Service Centre or a Gas Safe registered engineer. Checking the flue is engineer work.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
A05 Fan problem
- What it means
- The fan’s speed signal has been lost for an hour.
- Likely cause
- The fan or its wiring.
- What you can safely check
Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.
- Press the RESET button for 1 second.
- When to call a Gas Safe registered engineer
- If it remains after two resets, contact the Ferroli Service Centre or a Gas Safe registered engineer. The fan and wiring are for the engineer.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
A06 No flame after ignition
- What it means
- No flame after the ignition phase, six times in four minutes.
- Likely cause
- The detection electrode; an unstable flame; the gas valve offset; a blocked flue gas circuit; a blocked siphon.
- What you can safely check
Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.
- Press the RESET button for 1 second.
- When to call a Gas Safe registered engineer
- If it remains after two resets, contact the Ferroli Service Centre or a Gas Safe registered engineer. Every check on Ferroli’s list is inside the boiler.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
A41 Flow sensor disconnected
- What it means
- The flow sensor is disconnected.
- Likely cause
- The flow sensor’s position or connection.
- What you can safely check
Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.
- Press the RESET button for 1 second.
- When to call a Gas Safe registered engineer
- If it remains after two resets, contact the Ferroli Service Centre or a Gas Safe registered engineer. The sensor is for the engineer.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
F07 Flue gas too hot
- What it means
- The flue gas temperature has been above 95 °C for more than 2 minutes. Temporary; three times in 24 hours becomes A04.
- Likely cause
- The flue.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer If it keeps coming back, call a Gas Safe registered engineer to check the flue.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
F10 Flow sensor fault
- What it means
- A fault in the flow sensor: damaged, short-circuited or its wiring broken. Temporary.
- Likely cause
- The sensor or its wiring.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer to check the wiring or change the sensor.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
F11 Return sensor fault
- What it means
- A fault in the return sensor: damaged, or its wiring shorted or broken. Temporary.
- Likely cause
- The sensor or its wiring.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer to check the wiring or change the sensor.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
F13 Flue sensor fault
- What it means
- A fault in the flue gas sensor: damaged, or its wiring shorted or broken. Temporary.
- Likely cause
- The sensor or its wiring.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer to check the wiring or change the sensor.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
F14 Flow sensor fault
- What it means
- A fault in the flow sensor: damaged, short-circuited or its wiring broken. Temporary. (The boiler has a double safety-and-heating sensor on the flow.)
- Likely cause
- The sensor or its wiring.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer to check the wiring or change the sensor.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
F15 Fan problem
- What it means
- The fan’s speed signal is interrupted. Temporary.
- Likely cause
- The fan connection, a damaged fan or debris in it.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer to check the wiring and the fan.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
F34 Supply voltage low
- What it means
- The mains supply voltage is under 180 V. Temporary.
- Likely cause
- A fault in the electricity supply.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer If it persists, have an electrician check the electrical supply.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
F35 Irregular mains frequency
- What it means
- The mains frequency is irregular. Temporary.
- Likely cause
- A fault in the electricity supply.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer If it persists, have an electrician check the electrical supply.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
F42 Flow sensor fault
- What it means
- The flow sensor is damaged. Temporary.
- Likely cause
- A damaged sensor.
- What you can safely check
Nothing for the owner to check: this one is for a Gas Safe registered engineer.
- When to call a Gas Safe registered engineer
- Gas Safe engineer Call a Gas Safe registered engineer to change the sensor.
Source: Ferroli, Optimax HE Plus 18 OV instructions for use, installation and maintenance (cod. 3540M164, 02/2009): 1.6 faults, 3.4 troubleshooting Checked 2026-09-29.
A code that is not listed here
Other Ferroli boilers use other lists — sealed-system models, for example, have a water pressure code this open-vent model does not. Ferroli publishes each model’s manual on ferroli.com.
We list only codes the source below states. If your code is not here, it may belong to a model the source does not cover: your model’s manual is the authority.
Related pages
Source
Every code on this page comes from this page, published by Ferroli and read on the date shown. Meanings are written in our own words; nothing is taken from other makes, other models or third-party code sites.