I have a 1999 Forester with 112,900 miles. My mechanic just told me that I need to have two O2 sensors (front and rear) replaced (causing the "check engine" light). The catalytic converter was replaced in 2002. Is it normal for these sensors to go bad? How much damage will it cause if I don't get them replaced for awhile - I got a quote of over $500 for parts and labor, which I don't have right now.