If you live in a state with emission standards, you should have thresholds printed directly on the emission test results for OBD systems. My OBD-II does not print thresholds, however. It only shows values from my actual test. The chart excerpt below shows some typical cut-off values. Feel free to read the actual article to understand more about what the chart's values mean.
excerpt from: http://www.aa1car.com/library/tr1196.htm wrote:EMISSIONS CUT POINT CHART
Model year...Typical Cut Points...Well-tuned engine
..............CO%.....HC ppm..........CO%.....HC ppm
pre-1968..... 7.5-12.5... 750-2000..... 2.0-3.0... 250-500
1969-70..... 7.0-11.0... 650-1250..... 1.5-2.5... 200-300
1971-74..... 5.0-9.0... 425-1200..... 1.0-1.5... 100-200
1975-79..... 3.0-6.5... 300-650..... 0.5-1.0... 50-100
1980........ 1.5-3.5... 275-600..... 0.3-1.0... 50-100
1981-93..... 1.0-2.5... 200-300..... 0.0-0.5... 10-50
1994 & up..... 1.0-1.5... 50-100..... 0.0-0.2... 02-20
Below are actual Emissions Test Results for my two cars. One is older, from 1994 with no OBD-II connection and has thresholds. The other is a newer 2007 with OBD-II connection and has only pass or fail. Finding threshold values for the state of Georgia turned up nothing. It seems they might be a secret. haha


As previously stated, these numbers do not really translate to in-game numbers. Obviously, however, the closer you get to 0.00, the better your emissions! I aim for under 80 on my newer engines, and for really hard sport tuned engines, 120-150 seems more reasonable.