Premium? I think that would create more deposits given that it won't burn as fully.
But, make sure the car is hot, and let it sit for as little as possible before getting tested (ie, try to drive straight into the inspection bay w/o waiting to keep the cats hot). If that doesn't work, try new plugs, cap, rotor and 02 sensors.
finally, to gurantee a pass, pull an injector.
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