Auto is put on a Dyno and with a cone and some sort of probe inserted in the tailpipes. Tech watches a computer screen and increase speed based on the computer screen... Sort of like a video game. Car must stay between the lines and go up and down hills... ie have the car in gear reving to whatever will keep it inbetween the two lines. Test results HC - hydro carbons were fine...MA limit is 1.20 and it blew a .94 and a .96. NOx is the problem, limit is 2.50, first time it blew a 4.77, then i had the engine wire harness replaced. which corrected the issue with the EGR valve and the Air Pump, and the air intake sensor. Second test was much better, but still blew a 2.69. Which is a bit over the legal limit of 2.50. thanks for you help