In Maricopa and Pima Counties of Arizona, an inspection of light diesels has been in place for many years, since at least the late 1980s. The inspection may include certain northern, urban areas of Pinal County. Tucson is in Pima County. Metro Phoenix-Glendale-Mesa-Tempe is in Maricopa County, with a part of the far southeast valley in Pinal County.
I can only speak to what I have to get done. For my 1996 5 speed 4X4 3/4 ton truck, it's a snap idle test and an inspection under hood and of the fuel filler caps. (More about the cap inspection below. ) That's true for older diesels. I can't say what's done with newer models, if it's different.
I have had to get the snap idle opacity test done every year since I bought my truck new in 1996. (It may have just been a simple idle test for the first few years, but I can't recall for sure. ) It's one of the few tests where the state's emission testing contractor allows the owner to remain in the cab and actually control the fuel pedal. Actually, the owner must do the "foot work. " I suspect it is a liability issue.
The state's contractor has several testing stations scattered at fairly convenient locations. Depending on the local demographics, the multi-lane indoor testing stations will have one or two diesel testing bays. The contractor only does inspections. You pass or you don't. The state's contractor cannot do repairs, and thus has no financial incentive to artificially fail you and offer you a discount on repairs, then guarantee that you pass *only* if you get the repairs done there. If you fail, you get one free retest after you get the problem fixed. You can fix the problem yourself, or pay a facility to have it done. My truck has always passed.
The way it works is pretty simple. I drive onto the facility and drive up to what looks a lot like an airport parking lot gate. I push a button on the machine, and it gives me a bar coded ticket. The gate rises and I drive to the diesel lane. Sometimes I have to wait as much as 30 minutes. Usually, it's ten minutes or less. I get the inspection done in May or June, so the temperature outside is usually between 90F and 105F. Signs on the inspection building warn against overheating from idling, and idling with the air conditioning on. I find that my truck passes with a lower opacity if I let it idle with the air conditioning on. I augment that by turning on my exhaust brake to keep the EGT up. As I approach the head of the line, I lightly rest my foot on the fuel pedal to raise the idle to about 1,000 RPM to increase the EGTs a bit more. I have a 10 inch electric pusher fan just in front of the air conditioning condenser, which improves the air conditioning performance and overall cooling in stop and go summer driving in Phoenix. My truck has never even come close to overheating before or during the test.
When I reach the head of the line, I shut off the engine and the attendant (who is unfailingly polite) takes the bar coded ticket and my registration card. He or she compares the various VINs on the dashboard and door with the license number, and I hand them $28. They brief me on the test. Then the attendant inspects the fuel filler caps. I can't figure out why they do that, since the fuel tank system is vented, but I have never asked. The only time the caps ever got a second look was when the attendant was a Dodge enthusiast, and he had never seen the two-filler Transfer Flow aft-axle setup that I use. (Which, by the way, came with a CARB tag and number. Good job, Transfer Flow!)
One attendant wheels over an immense fan to blow air over the radiator, as if I was driving at about 40 MPH. The attendants then get out a nozzle and a hose large enough to go over the tailpipe. Then they hook up a test device that looks a lot like the wacky robots at Disney's Star Tours. They replace some sort of filter in the test sensor, and then hook the whole thing up while I sit in the cab.
Next comes the governor test. In neutral, I am instructed to press the fuel pedal to the floor to make sure that the governor keeps the engine below redline. I think that this part of the test is why the emissions contractor wants the owner in the driver seat.
The snap idle test follows. There is a video screen which will say "accelerate to maximum" and after a short time, it will say "return to idle. " There will be several cycles of this. The first few cycles will be sort of a dry run, in case there has been a cylinder temperature reduction while waiting. Then the real test will follow, usually after the engine has fully warmed up again. If the engine is running really clean, the dry run will serve as the actual test, and it will be over quickly. I've found that if get the inspection on a hot day, and if I idle with the air conditioning on, as I described above, I usually finish during the dry run.
There are occasional problems. None of the problems have caused me to fail, but they are amusing. If I follow, say, a fairly late model VW or Mercedes diesel, my truck's printout shows a very low opacity result. On one occasion, I followed an ancient calico-colored Ford diesel that was missing various body bits, and lightly smoking black at idle. That truck had Oklahoma license plates, a sooty bumper, and was obviously getting its first emissions test ever. It took several tries, and I don't know if the Ford passed at all. I know that the attendants cleaned out the sensors and put in new filters, but that test showed the highest Opacity result my truck ever had. It still passed, though. I might have been less than amused if my truck was high-mileage with marginal injectors, and that little extra carbon had put my test over the top.
I also find that when I am using the Cummins-Valvoline diesel additive sold by Geno's, the truck performs noticeably better on the test.
We lived in California when gasoline car testing began in about 1982. Then it was done by the same places that wanted to sell me a tune-up. I had a good relationship with an honest place in Riverside, so there I never had any trouble. When we moved to Arizona in 1985, we had to get all our cars tested every year. At first I was annoyed by it. But later I learned how to interpret the printed out test results and I was able to use them as a sort of condition report for how well my cars were running. The tests weren't terribly expensive. They still aren't.
My wife's 2007 Chrysler Town & Country with OBD II emission controls won't need an inspection for several years. I can't recall the exact number. Our 2001 PT Cruiser gets them every two years, I think. Our late 1993 Saturn SW2 got tested every year, and passed every year.
The bottom line is that I've never had a problem. I take something to read and just people-watch. But my truck is bone-stock and unmodified except for the aft-axle fuel tank, the Prime-Loc fuel filter relocation kit, and the exhaust brake. I've been told that turning up the fuel pump can cause snap idle problems, depending on how great the change has been.
As always, your mileage may vary. To sort of paraphrase Bob Patton--with apologies--in Arizona you can't become your own emissions station.
Attention: TDR Forum Junkies 
If you know how to drive you can pass with no problem. My good friend called me this fall saying he had failed. After some coaching and a 2x4 under the go pedal he went back and passed with flying colors. 
