Ford Taurus Emmissions Test

There are a couple of codes for different types of vapor leakage, depending on the severity of the leak, there's a so-called gross leak, then there's a minor type, which usually is the gas cap, unless the seal is so bad that during the self-test, it's contrued as a major(gross) leak. The gas cap is part of the evap system test, which checks your fuel system for vapor leakage, I believe every cold start, I could be wrong on the frequency, just guessing without consulting the book. Some systems use vacuum, some use a pump to pressurize the lines and then check them for leakdown, slow fast whatever. What is NTB?

Is there some state required place you're supposed to take the car to? Or any approved station? Take it elsewhere, usually the gas cap is the problem, not always though, when there is a minor leakage issue. I don't get the testing place refusing to re-test.