Final chapter on this saga: burned two tanks of 91 Chevron premium per GM instruction. Also, per emission mechanic's private advice, ran it up and down some hills on the freeway in 2nd gear at high RPM day before test. Returned for re-test yesterday a.m. w/fingers crossed. Passed with flying colors. NOx reduced from about 900 down to 78! No new cat (or anything else) neccessary. Now, back to regular gas....