What do you do if you by a used car that won't pass emissions?

What do you do if you by a used car that won't pass emissions?
If you bought it "as is" and untested you will have to check in your area to see if you have a Lemon law that allows you to return the vehicle. Usually "as is" means you have no option other that fix it at your expense. What did it fail on the emissions? What were the limits & resulting numbers on the test for HC, CO & NOX? Maybe it's a simple fix to get it to pass. Year/make/model/ engine size/ mileage & symptoms (if any) will help as well as any maintenance history you know of.