Something has been going on for a few weeks. I've noticed backfiring on almost all of my 1st and 2nd gear shifts, and about 50% of the 3rd gear shifts. It feels as I've lost a bit of power, and my MPGs have dropped. <p>I would think at 40k miles its a bit early to suspect O2 sensor malfunction. I have no codes, no CEL. <p>All lines appear connected.