Help - check engine light on

My old altima stalled before red light agian today. However, this time the check engine light was on.
I went to autozone and pulled out the code. Surprisingly I got two - knock sensor malfunction and gear 3 ratio incorrect.
I reset the code and drove home. The check engine light is off now. It seems that the problem is intermittent.
My car is a 97 altima, 92k miles, a/t transimission.
Do I need to replace the knock sensor? Is it a safety issue? How to fix "gear 3 ratio incorrect"?

Thanks.