Here's my take on e-check.
On our '95 Mustangs, each one required a passing e check certificate to obtain the collector's plates. My husband's 95 is a Kenne Bell blown car. His car passed with minimal readings. My '95 had heads, cam, intake, and all the smog equipment removed, except for the cats. The car passed with a reading three times as high as the KB car. Now, our '89........we had it tested once bone stock. The readings came out to around 24%. Took the car back 2 years later (June 2005) with heads, cam, intake, and every bolt on known to man. The result? 3% pollution. That's why I swear up and down it's "fixed".......no way in hell does that '89 run cleaner now than it did bone stock. And mileage isn't a factor. It had 15,300 original the first time around, and now has 18,000.
95 Mustang GT - finally happy with it
89 Mustang LX - fat slug, but it runs awesome
94 Thunderbird SC - For sale