I just took my 1990 chevy Silverado, 350, 2WD, to the Ohio emissions check station. It failed! Supposedly the Nitrous Oxide level was too high. Also the gas cap didn't hold pressure. Now I have a headache to resolve. They gave me a new gas cap for free, so that's supposedly taken care of. My catalytic was replaced about 7 months ago with an Autozone unit. (Not sure if I have faith in the unit.) Prior to the catalytic change, my truck's reading were always much, much lower than the state's maximun level. Nevertheless, I have to deal with this problem. I'm supposed to repair this problem and then take the truck back for another check. My question is how can I check my emissions, after I make repairs but before I go back to the state, so that I know before hand how my emissions are doing. Is there any cheap way to check my emissions without going to an E-Check station? Also, this would also give me a second opinion on the status of my emissions. My research shows that the EGR valve may be the culpret. And possibly the EGR solenoid vlv. Anyone deal with this problem?