i would recommend hooking up the air temp sensor. it just think that its there for a reason, even if you have a k&n the ecu would need to detect the air temp just to get the best mixture.
have you tried checking the code not using a code reader? im looking in the chiltons and it doesnt list the first one. goto
www.240sx.org and goto the faq and they tell you a way to check the codes withouth a code reader. then post the codes.