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
Below is a video demonstrating a basic maintenance procedure on a Noritz tankless.
The procedures shown in this video should be performed by a trained service professional as it deals with gas, water and electricity within confined spaces.
If the unit is supplied with hard water and a softener or water treatment is not installed, the unit will need to be flushed annually to clean up and remove scale deposits that have formed inside the heat exchanger. Check out this article for the descale procedure.