Low
39 on Ferroli Optimax

Your boiler has paused temporarily because it detected an unusual reading, likely from the outdoor temperature sensor, and will restart itself once the reading returns to normal.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description

Temporary shutdown fault - resets automatically as soon as the value comes back within the boiler’s normal working range

DIY fix possible

Check if the outdoor temperature sensor is covered in snow or ice and gently clear itWait for the boiler to automatically restart when the outside temperature stabilisesMonitor the boiler to see if the fault clears without intervention

Find a Gas Safe engineer
Search the official register →

Related fault codes

F39View →F37View →

Related guides you might find useful

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →

New boiler cost 2026

Realistic UK prices for a swap.

Read →