Chrysler Repair: dodge neon: codes 12,21,43, powertrain control module, obd ii code reader

Question
my dodge neon is running a little rough i got codes 12 43 21

Answer
Hi Troy,
The 21 code says that either the upstream or downstream oxygen sensor is not changing its voltage reading. It is likely that a readout using the underdash plug and an OBD-II code reader will give you a more specific code telling you which sensor is bad (or possibly there is a defective signal wire connection between that sensor and the powertrain control module). If you live near an Autozone parts store they will do a free code readout and be able to tell you which sensor is the problem one.
The 43 code just tells you what you already have noted, that one or more of the cylinders is misfiring. This may be due to the oxygen sensor problem, so deal with that one first, then see if the missing is remedied.
The 12 code just says that the battery was disconnected sometime in the past 50-100 key on-off cycles.
Roland