Hi John:
I believe that you might have a problem with the test because Volvo has one of the OBD systems that "wipes out" stored emissions information if the OBD codes are purged or if the battery is disconnected for service. Apparently the test requires you to have a certain amount of "history" in your OBD II system to read how well the engine is performing. I don't know why the speed sensor was failed the first time, but I suspect the second time is because of the cleared codes.

This was recently publicized as a problem in NC, and, of course, Volvo was one of the types of cars listed as having thí troublesome OBD. Here is an article from the Charlotte Observer that describes the problem:
http://www.charlotte.com/mld/observer/news/local/4875013.htm

There was some solution that was proposed for the immediate term (like going to the dealer and getting your car "certified"). Don't remember what it was. But it was clear that they need to do something about this issue.

Hope this was helpful.

Mike Brown
Greenville, NC
1996 965 92K
1986 244 132K