The washer liquid tank capacity is actually 5L and the low level light comes on when there is 1L left in the tank.
The default behaviour of the system is:
- With the main beam on when spraying the windscreen the first time, the headlight washers will activate as well.
- After the first activation, each 5th windscreen wash will activate the headlight washers cycle as well.
- When the liquid level reaches 1L, the low level light comes on the dash and the headlight washers are disabled to save the remaining liquid for the windscreen. Once topped up, the washers are automatically re-enabled.
This behaviour can be changed with VCDS both the number of windscreen washes between two headlight wash cycles and the delay in seconds for first wash which by default is set to 0s (hence the washers cycle will begin as soon as you wash the windscreen the first time.)
In your case either the default behaviour has been changed or you have an issue with the electronic module that drives the washers or maybe a cabling issue. Either way a trip to the dealer is required.