Chrysler Repair: 1997 2,5 V-6, code 21, thread chaser, oxygen sensors

Question
i just got a code 12 and 21 any ideas

Answer
HI Scott,
The 12 just says the battery was disconnected in the recent past, which unless is untrue I would ignore. The 21 says one of the two oxygen sensors is not showing any voltage variation. Probably it is the upstream sensor located on one or the other of the two exhaust manifolds (looks like a spark plug). Just in case though take a look at the wires on the downstream sensor at the catalytic converter.  
Soak the threads overnight with some penetrating oil, then warm up the engine, and finally gently rock it back and forth to loosen up the threads and remove. Be very careful not to damage wires on new unit. There is a special socket for installing/removing just for oxy sensors, but you may not have to use it depending upon the other sockets/box end wrenches that you have. Tighten it to about the same as a spark plug. Use the grease that comes with the new one before installing. If the threads seemed rough then buy a thread chaser before installing the new one.
Roland