Got an 04 Acura TSX that's been baffling me for the couple of weeks. Accidentally bought an Upstream Oxygen sensor instead of the downstream so I thought might as well replace it since I have it (keep in mind the Upstream (Air/Fuel) Oxygen sensor worked fine before and the Check Engine light was for the downstream.
WORST IDEA EVER! Car ran very poorly! Ran lean and spark plugs turned black. Car would shudder as if it's going to stall. RPMs would go from 800 - 1100 and fluctuate constantly and this lasts 5 seconds then everything is fine. It happens when I change gear and when stopped at a redlight. I've replaced the Spark Plugs, cleaned the Mass air flow sensor, and put a new OEM Denso sensor. Still having this issue but this time it's not running lean but did pop-up these codes:
Both codes are referencing Bank 1 Sensor1 which is the sensor I just replaced.