It is easy to extract the codes on a '91. You will have a two pin connector under the dash on the passenger side just above the kick panel. You can short out that connector with a paper clip and turn the ignition to on (not run). Then the computer will flash the codes to you. Short duration flashes are a single digit code; long are a second digit. I.E., three quick flashes is a code 3, Two long flashes and 1 short would be a 21. Multiple codes are separated by a long pause in between. You can do a search on google for engine codes and get all the code info..