What it means
Many water heaters have an automatic frost-protection valve that opens and drains the tank if internal temperature drops close to freezing - a fault light can mean this has triggered, or that the unit itself has a genuine fault.
What to do
- Check whether the frost valve has opened and drained the tank - this is often normal, protective behaviour, not a fault.
- Reset the unit per the appliance's own instructions and see if the fault clears.
- If the light persists after a reset, book it in for a proper look before relying on hot water again.
Likely causes
- Automatic frost protection doing exactly what it's designed to do
- A genuine element or thermostat fault
- Low battery voltage affecting the control board
Rough UK cost
Often no cost if it's just the frost valve resetting; unit repairs run £80 - £300.