For those who face the dreaded so called Enhanced Emission test here are a few hard learned tips. 95 and earlier vehicles have many problems passing this test, so much so the manufacturers usually have tech help lines for dealers to get them through. My first lesson was last year with my 91 Limina Z-34. Runs fine, good fuel economy. The only change I made was replacing the factory mufflers with low restriction units. It failed miserably. The year before it went through fine. Lesson #1, in most cases your 95 and earlier vehicle will fail if you use a low restriction exhaust. Many times with high flow exhaust the cat will not light up enough to pass the test. Lesson #2, the ECM's were not as good at compensating for different running conditions as the newer PCM's are. Last 2 weeks were spent with the 89 Iroc. It failed time after time for I/M 240 specs. Since everything was replaced, EGR and solenoid, O2 sensor and cat, I was stumped. Tip #1, find a guy who will work with you if it fails. Most decent mechanics see this everyday with all sorts of cars so it's not unusual for a car to fail on the first pass. Tip #2, and this is right from GM-make sure before it is tested that you run the rpm's up in park/neutral to 2500-3000 for 3 minutes. If the car idles for more than 2 minutes insist that it be done before testing. This can make the difference between passing and failing. Tip #3, if you are blowing on the high side for CO, HC and NO and have an aftermarket high flow exhaust expect to fail. You will need a stock muffler. Tip #4, when setting base timing make sure the car is hot-not by what the temp reads but really hot after a good run. One thing I noticed is that the ignition modules drift with heat. I put a timing light on mine after idling in the driveway, pulled the EST connector and it was right on 6 deg. I took it out for errands, came back and did the same test and it had drifted to almost 8. Make sure the dist base is good and hot then set base timing. Tip #5, if you have some mods simply disconnect the EST and have it tested. In most cases it will pass. Fact is, I/M 240 was designed to get cars off the road that are not absolutely perfect. The specs are so tight that just about anything you do to improve performance will put you out of tolerance. Hope this helps some you unfortunate people that need the test.