Low Information
S.21 on Glow-worm Ultracom sxi

Your boiler is performing a routine check of the internal fan system before it begins the heating process.

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

fan pre-run

What causes this fault?

This code appears when the boiler's control board sends a signal to the fan to spin up and clear any remaining gases from the combustion chamber. It is a standard safety procedure to ensure the system is clear and safe before the burner ignites. If the code stays on the screen for a long time, it suggests the fan is struggling to reach the required speed to allow the ignition sequence to continue.

DIY fix possible

No action is required as this is a normal status message.Wait a few moments for the boiler to complete its startup sequence.If the code stays on screen for a long time without the boiler firing, try a simple reset.

Find a Gas Safe engineer
Search the official register →

Related guides you might find useful

Banging / kettling / whistling noise

Each noise points to a different fault.

Read →

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 →