To measure nanomolar levels of NO, as done by many researchers, the user needs to have better grounding.  Fluctuations in environmental noise and temperature will effect background. The meter should be grounded and not placed under an air condition vent or where the flow of air is high. The use of a Faraday cage will help, (a poor mans version can be constructed from a cardboard box covered with aluminum foil then grounded).

Another solution is to user better filtering: Go to SETUP then FILTERS then choose 10 orders, for heavier electronic noise filtration. This is found in version 2.2 of the software.