External sensor fault
The F39 error code on Ferroli boilers indicates external sensor fault.
Ferroli F39 means: external sensor fault. The usual causes are sensor damaged or wiring short circuit or sensor disconnected after activating the sliding temperature. A Gas Safe registered engineer should diagnose the root cause.
What Does F39 Mean
The F39 code is shown by the boiler’s control board when it detects external sensor fault. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows F39 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in the Ferroli service literature published by centriassistenza.org, cross-checked against the manufacturer’s fault-code table.
| Cause |
|---|
| sensor damaged or wiring short circuit |
| sensor disconnected after activating the sliding temperature |
Safety Warning
F39 is a gas-related fault. Never attempt to repair or adjust the gas valve, burner assembly, or flue system yourself. All gas work must be carried out by a Gas Safe registered engineer (UK) or licensed HVAC technician (US). Repeated resets after ignition failure can cause unburnt gas to accumulate inside the combustion chamber — this is a serious safety hazard. If you smell gas at any point, turn off the gas supply at the meter, open windows, leave the property, and call your gas emergency service immediately. Warning: this fault code may relate to gas supply, flue or combustion issues. Do not attempt repair — contact a Gas Safe registered engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
1. Note the exact error code and check the manufacturer's installation and service manual for specific guidance. 2. Check all visible wiring connections for security and condition. 3. Power cycle the boiler — switch off at the mains for 30 seconds, then switch back on. 4. If the fault recurs, check the fault history for related or recurring codes. 5. Inspect the boiler for any obvious signs of damage, leaks, or unusual sounds
How to Fix It
Check the wiring or replace the sensor
Reconnect the external sensor or disable the sliding temperature
Refer to the Ferroli service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Ferroli models that can display fault code F39:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Ferroli models | — | Possible |
Frequently Asked Questions
Check the wiring or replace the sensor. According to the technical documentation, this is the first check on the list for F39.
F39 is classified as a high-severity fault: it involves gas, combustion or flue components. Do not keep resetting the boiler and do not attempt the repair yourself — call a Gas Safe registered engineer.