I also drive a 91 Civic DX and you will need to rip up the carpet in order to count codes on your ecu...just pull back the carpet on the passenger side (the area where they put their feet under the glove box) and you will se a metal plate with a little glass window on it...you will see a red flashing light and that is what you need to count in order to see what code it is...go to this site to see what the codes mean for your car...
http://www.fourthgenhatch.com/helms-ecucodes.html

hope this helps