Last time I had to pass an emissions test in a gas powered vehicle, I found out that going for a nice long highway drive for at least an hour really helps. It's better than just letting it sit and idle for an hour. You have to drive at highway speeds for at least an hour then immediately go for your emissions test.
Again, that worked for gassers, not sure if it would help a diesel.
Again, that worked for gassers, not sure if it would help a diesel.