90MUSTANG

I pre-tested (California) my 1990 Ford Mustang LX 5.0 and it passed everything except the "25 MPH" part of the test in which it failed the NO part of the test. What can I do to get it to read lower on that part of the smog check? I have replaced the EGR valve after I failed the test but I am not sure if that is going to do the trick. The timing is correct and the plugs,wires,rotor,dist cap,air filter have been replaced. The smog guy told me that the EGR valve was not getting enough air or something but when I put my finger over the plug, I can feel it "sucking" air after releasing my finger. Do you think the O2 sensors need replacing to make it pass? After I came back to him with the new EGR valve he said that "I wasted my money" and that for $80.00 labor+parts he would fix it and make it pass. The old EGR valve had lots of "black" carbon inside of it and I am sure it was the original part from 1990. Any help would be great, Thanks...