Quite awhile ago I found the procedure for how they do the test.
It's been awhile, so my time intervals are probably wrong, but there are 2 dyno tests. (15 & 25)
For each test you have up to 300 seconds to pass. The 300 seconds are divided into 30 second bins.
The ECU adjusts A/F ratio based on the first sensor. The second sensor measures the effectiveness of the cat.
Didn't think a 91 had 2 sensors...…...…...…..
Once you pass a bin, you've passed.
So if the cat is warming up during the test, each bin would be getting better than the previous bin. You got under the limit for a bin...….. test over you passed.