Error code 16 isn't list in the owners' manual - call Noritz. It's under warranty- they should be willing to talk you through the diagnostic, or refer you to local technical support.
Have had this tankless system less than 6 months. This weekend it began to flash "16". We've cleaned filter and turned on/off. Will get hot water for a few minutes, but then the 16 starts again. Would appreciate any ideas!