Low
d.16 on Vaillant Home System

This is a status indicator rather than a fault, showing whether your external heating controls are currently asking the boiler to turn on or stay off.

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

Status of the 24V room thermostat: off=Heating off, on=Heating on

DIY fix possible

Check your room thermostat is turned up higher than the current room temperatureEnsure your programmer or heating app is set to an 'On' periodReplace the batteries in your wireless room thermostat if the screen is blank

Find a Gas Safe engineer
Search the official register →

Related fault codes

S.0View →d.18View →

Related guides you might find useful

Boiler won't turn on — 9-step checklist

Work through these before the call-out.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →