Changed out the upstream O2 sensors on my 2003 XK8 today, in response to a P0153 DTC. Although left, driver-side unit indicated, changed out both, as recommended. Apparently solved the problem, as code not recurring. Spent as much time trying to find a good quality offset O2 sensor wrench ("crowfoot sensor wrench", i.e. offset socket wrench with a slot in the side to pass the connector cable. Also make a standard deep-set socket with a side slot, but not long enough unless you cut off the end of the sensor - more work - and you don't have room to get a ratchet on the end of it.). The auto parts stores here locally are selling a generic 22mm, 7/8ths socket that is very inaccurate - measures nearly 23mm and will simply round off the hex-base of the sensor rather than loosen it. Finally found a "Great Neck" 22mm wrench packaged for sale by "OEM Tools" that measured a true 22mm and fit a 1/2" drive. May seem to be a minor issue, but realize it will take an 18" breaker bar to remove the original sensors (mine in for 110k miles). Would try to find a true 22mm rather than 7/8". Although only 1/100 inch difference, you need all the help possible when applying 100+ pounds of torque. Take a metric ruler along when looking locally or order a 22mm on line.