All police crown Vic's are dual exhaust, you got it under single and if reading was only taken from one pipe the car should be retested, I just did one last week and I was able to do a dyno test.
The exhaust had an "X" after the catalytic converters(equalizer,crossover whatever you want to call it)so the exhaust is distributed equally. What I was told, that is considered SINGLE exhaust so I tested it single.
It's a bad example. This car should not be tested on the dyno.
As far as emission failure with CEL off: yes that happens. May be 1-3% of the older OBD2 cars from my experience, and BAR is willing to accept this since it is very normal margin of error.
It may be 1 to 3 % now, but in the future it will increase, because it is going to be very very easy to get a polluting vehicle to pass a OBD only test. They already have hand held devices to clear the computer and set all monitors to ready. It can be done in the parking lot. You can not change tail pipe emissions in the parking lot. So in time all the hard work that has been done over the years will be for nothing. All the crown vics show not do dyno test, the vehicle i mentioned earlier was a crown vic not sure the year, but at the time of the test we were testing egr valves, the current law says 1996 and newer we don't have to test on two speed idle tests. When I asked a pr2 about it, he said always go by the book. Well the problem is not all books are created equally and they may change information from year to year.