My 91 420 sel failed the NOx portion of the emissions test. All the other emissions were well within specified limits. Does anyone know which emission related component is most responsible for the control of NOx? I suspect that the problem is related to the EGR system. How do I approach the problem and figure out what needs to be fixed?

My test results were:

allowed actual
---------- --------
HC 103 13 (hydrocarbons)
CO .57 .05 (carbon monoxide)
CO2 na 14.7 (carbon dioxide)
O2 na .1 (oxygen)
NOx 712 924 (nitrogen oxide) FAILED

__________________
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k