i am coding a 12 and 21....are these related? and what should i check first? im gonna start workin on it now but i figured i would post so i can have a idea whats next if i get stuck. thanks

 

Error code 12 is nothing except that you have disconnected the battery within the last 50 key on-off cycles.

Error code 21 is your O2 sensor (usually upsteam) going/gone bad.

The P codes on the PCM will tell you which one. Go to a local parts store and get them to give you a free readout.

The upstream sensor is on the exhaust manifold just under the air box. Remove it and there it is. The downstream sensor is behind the CAT. Borrow the O2 sensor tool from the parts store to make it easier to remove and replace it - if you don't have one already.

 

BTW, check the wires to make sure that you don't have a worn spot that may be causing a short. It seems that there have been a rash of these lately and most center around the O2 Sensor wires. Usually these appear anywhere there is a pinch point (wire tie, sharp edge) or where the wires are banging around because they are bouncing around and contacting metal.