Hi I have a 2001 Nissan Pathfinder. Ive been getting my service engine light repeadtly come on. One time its the o2 sensors next time the code reads low fuel flow. There was an aftermarket exhaust put in and nissan is saying put back to stock and that costs 1000 dollars but my mech says that an aftermarket exhaust shouldnt cause a problem. My service engine light is still on and there is a rattling sound or a sound of rocks shaking around. Im wondering if it can be the catalayc convertor. Please can someone help me solve this problem! Thanks!