cant pass emissions test

i have 1996 maxima with almost 119,000 miles. it has been a good running car and i have kept regular maintenance on it done. for 1 month now, i have been trying to get my car inspected. i keep getting these 3 messages: "catalytic convertor efficiency, egr flow, and evaporative emissions control system" not ready. i have had the egr cleaned, spark plugs, and thermostat changed. the hand held computer says that there are no codes. the mechanics keep telling me to drive the car for about 30 miles and then come back but when i do it still won't pass. the nissan place said the o2 was bad but on the test, it says that the o2 sensors are ready. I'm totally lost as what to do except to keep "trying" to get things fixed one at a time until something is right. thanks for any help that you can give me.