Since your '96 is OBD-2, maybe your mechanic plugged in an OBD-2 scanner. The error numbers are different vs. the blinking-light method.

P0300 = random misfire
P0301 = 71 = cyl-1 misfire
P0302 = 72 = cyl-2 misfire
P0303 = 73 = cyl-3 misfire
P0304 = 74 = cyl-4 misfire
I guess these can also be indicated by P1399.