Today my Z28 failed emisssions and the car is pretty much stock except for headers, CAI, and 1.6 rockers. The car runs fine with no SES light and my Scanmaster shows no codes and all the readings seem to be fine also.

Here is what failed.

NOx = 887 @ 1800 rpm @ 25 mph. Should not be over 796
NOx = 1299 @ 1800 rpm @ 15 mph. Should not be over 879

HC = 123 @ 1800 rpm @ 25 mph. Should not be over 114
HC = 162 @ 1800 rpm @ 15 mph. Should not be over 117

Items that were replaced this summer were both O2 sensors, EGR valve, and catalytic converter. In the spring I had heads reworked while repairing a head gasket. Any input would be greatly appreciated.

 

Have you changed your plugs lately? Use premium fuel. Get the car good and hot. Change your air filter. Make sure your MAF is clean (use electric contact cleaner on it). Fill your rear tires with 50 lbs of air for less drag. Change your oil. Check that EGR valve. I used denatured alcohol 1/2 gas mix and reduced my NOX emissions by 266 ppm. It didn't change the CO though which was interesting. The O2 sensors you got were heated right? Anyone else kick in some ideas. I would also go to a different testing facility.

 

When I failed my pre-test. I went back home and set my fans back to stock setting, use a stock spark plug heat range, pour two 8oz bottle of anti-freeze treatment which contains methanol into a 1/4 tank of gas to help get those numbers down. It pass with flying colors now.