Short story. My 2005 avalanche has recently began multiple misfiring at various times. Only at idle, never at speed. Shut the truck off. Start it up. Usually clears the problem up for a week or more. Throws random misfire code. Also has thrown a 02 sensor fault code twice. It has had a tuneup in the last month. Everything else works and checks out fine. Coming home from generous motors tonight. Ran fine. But ate a ton of gas. Filled up before I started my last leg home. Ran like crap, I knew it was misfiring, but it also misfired up to speed until I hit about 80 then smoothed out. Idiot light came on and actually stayed on. Not just flash at me and dissapear. Scanned it when I got home. 3 oxygen sensors have thrown codes. 2 are GM specific. Also a random misfire code. Exhaust smells like crap. I know the cat is good still for the time being as it smells fine when the truck is my acting up. With 250,000 km on the odometer. I am thinking they are original and they looked original when I installed my flowtech mufflers. I was gonna replace all 4 this weekend and see what it does. I don't have access to my genesis scanner for a few days so I cannot check the voltage and ohm and milliamp readings. But for 02 codes to have been thrown, twice before and now 3 out of 4 tossing codes tonight, what do you think?