When I was looking into a yellow light on the wife's car I got into this. She'd been driving it for many trips and a long time but the self-test still hadn't completed. It hadn't recorded an error - it just hadn't completed the tests.

The problem was her driving profile just didn't include the acceleration and speeds necessary for the self-tests. She rarely gets up to 45 mph and drives stoplight to stoplight with few periods of sustained speed. I drove it to my work and back one day (100 mi round trip) and they'd all completed.