Hard to answer, since most of the information about HEMP is classified, but also because the question is too broad. Any kind of EMP is not going to affect the same a car in an underground parking than in the open, one right below the EMP source than one 2,000 miles away, one 1988 car or a 2015 car, and so on.
As a general rule, a HEMP caused by a nuclear explosion in the stratosphere can stop dead most modern cars in a large area (hundreds of miles). However, many of them can be restarted (if they don't crash as a result) or be repaired without too much hassle. You can google for articles about EMP effects on cars, such as this one but they all refer to the same report as a primary source: this study from the EMP commission, which is, to the best of my knowlegde, the only publicly available study ever made on EMP in cars.
The test was made using vintage cars from 1986 to 2005, and since they had to return the car after the tests, cars were tested starting with weak EMPs and were stopped whenever a car showed evidences of being affected. They concluded that most cars won't be affected by EMPs at all, or only suffer minor glitches (lights blinking, radio failing), but about 10% of cars could have their engines halting if surprised on the road, potentially causing accidents; however, newer cars with much more electronics and microchips may be way more vulnerable than the vintage cars that were tested.