The filter, located on the cold water inlet fitting, should be checked for debris about once a year.
The front cover, fan, and venting should be checked for dust and debris. Depending on water quality, a heat exchanger flush may be necessary as well.
further detail can be found in the owners guide, located at http://support.noritz.com/technical-literature.php
Below is a video demonstrating a basic maintenance procedure on a Noritz tankless.