Ferroli A39 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli A39 fault code mean?
The A39 code indicates that the boiler has detected a problem with the external (outside air temperature) probe — an optional sensor that, when fitted, allows the boiler to modulate its output based on outdoor conditions, a feature known as weather compensation. The boiler has lost a valid signal from this sensor, either because it is damaged, disconnected, or was enabled in the boiler's configuration but never properly installed. Because A-prefix codes are lockout faults on Ferroli boilers, the boiler will shut down and require a manual reset to restart. It will not clear on its own.
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
- Loose, corroded, or disconnected sensor wiring Common
The external probe is mounted outdoors and its cable runs back into the boiler. Over time, the cable connection can work loose at either end, or the terminals can corrode due to moisture exposure — this is the most frequent trigger for A39.
- External probe enabled in settings but not connected Common
If weather compensation mode was activated in the boiler's configuration menu but the sensor was never physically installed, or was later removed without disabling the setting, the boiler will flag A39 because it expects a sensor signal that is not there.
- Failed or damaged sensor Sometimes
The probe itself can fail internally — presenting either an open circuit (no signal) or a short circuit (out-of-range signal). Outdoor exposure to frost, UV, or physical damage can shorten the sensor's life.
- Wiring damage from weather or pests Sometimes
Because the sensor cable runs outside, it can be chewed by pests, pinched during building work, or degraded by prolonged exposure to rain and freezing temperatures.
How to fix it
- Attempt a boiler reset DIY safe
Locate the reset button on your Ferroli boiler — it is usually marked with an 'R' or a flame symbol. Press and hold it for 5–10 seconds. If the fault clears and the boiler fires up normally, monitor it over the next few hours. If A39 returns, move on to the next step rather than repeatedly resetting.
- Visually inspect the external sensor and its cable DIY safe
The outside air temperature sensor is typically a small probe clipped or screwed to an external wall. Without opening the boiler casing, check whether the sensor is still physically in place and that its cable appears intact — no obvious cuts, chew marks, or sections pulled away from the wall. Note any visible damage to report to your engineer.
- Contact a Gas Safe registered engineer to inspect and reconfigure or replace the sensor Gas Safe engineer
If the fault returns after a reset, or if you can see cable or sensor damage, you need a Gas Safe engineer. They can: test the sensor's resistance to confirm whether it has failed; re-seat any loose wiring connections; disable the external probe setting in the boiler's configuration menu if weather compensation is no longer required (this is often the quickest permanent fix); or replace the sensor if it has genuinely failed.
Parts you may need
- Ferroli external outside air temperature sensor · from £25
- Replacement sensor cable/wiring loom · from £15
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 £80–£150, depending on the underlying cause.
Frequently asked questions
Can I just disable the external probe setting to clear the A39 fault?
Yes — if you no longer need or use weather compensation, a Gas Safe engineer can turn off the external probe function in the boiler's configuration menu. Once disabled, the boiler stops expecting a sensor signal and the A39 fault should clear permanently. This is often the cheapest and quickest resolution, covered within a standard call-out fee of roughly £80–£120.
Is the A39 fault dangerous?
Not directly. Unlike faults related to gas pressure, combustion, or flue gases, A39 is about an optional temperature sensor rather than a core safety system. However, you should still arrange a fix promptly — the boiler is in lockout and will not provide heating or hot water until the fault is resolved. As always, if you smell gas or notice any leak, leave the property and call the National Gas Emergency Service on 0800 111 999 immediately.
How much does it cost to fix a Ferroli A39 fault?
Because A39 involves an inexpensive optional sensor, it is one of the more affordable Ferroli fault repairs. A call-out, diagnosis, and configuration change (disabling the probe setting) typically costs £80–£150. If the sensor itself needs replacing, parts are cheap — usually under £30 — so an all-in replacement is often only marginally more. Labour rates vary by region, so always get a quote upfront.
My Ferroli boiler keeps returning to A39 after I reset it — what should I do?
A fault that keeps coming back after a reset almost always means the underlying problem has not been fixed. Stop resetting the boiler repeatedly and call a Gas Safe engineer to properly diagnose the cause — whether that is a failed sensor, damaged wiring, or a configuration setting that needs changing. Repeated lockout and restart cycles without resolving the root cause can place unnecessary stress on other boiler components.