Check the coolant level in the coolant expansion tank -1-.

WARNING: This page is about a different car, the 2017 Volkswagen Beetle. However, it is still accessible from the selected car via links, so may be relevant.

-- Check the coolant level in the coolant expansion tank -1-.

Fig 1: Identifying Coolant Reservoir

Courtesy of VOLKSWAGEN GROUP OF AMERICA, INC.

The coolant level must be between the "min" and "max" markings when the engine is cold.
The coolant level may be at the "max" marking -arrow- when the engine is warm.

-- Add more coolant if necessary.