Your test report should state the units of measure, however, a "negative" number is perplexing.

Emissions are usually measured in either PPM (parts per million) or percent. One PPM equals .0001 percent, so the unit of measure is usually chosen based on the sensitivity of the measuring equipment and to minimize the number of zeros relative to the significant digits so the data is easy to read. If your car is tested using the I/M 240 procedure the emissions are usually measured in grams per mile.

In California HC and NOx emissions are reported in PPM and CO and O2 are reported in percent; O2 is not an emission, but it's a very useful diagnostic tool to determine how well the emission control system is functioning.

A NOx reading of .02 percent is the same as 200 PPM using the above stated conversion, however, a negative number doesn't make any sense. Could it be that the "-" is a dash as opposed to an algebraic sign?

Duke