Along with my other problems, I am in dire need of a muffler, mine looks as if it got shot with buckshot. I do not know thee history of my O2 sensor either. My truck has 188K. Trying to boost fuel milage, I am getting 14-16 and just get it running good. My check engine light is on for my O2 sensor. Should I replace muffler first, and see if that corrects my O2 sensor reading, (read where a certain amount of back pressure is needed), and with ALL the hole I know I am not getting ANY backpressure. I priced a muffler and tailpipe assembly at Advance Auto for $100. Does anyone know of a cheaper or better quality for the money source for muffler/tailpipe assemblies?