code 12 and battery is good and connected

95-LHS Chysler Had trouble passing emmision test. Car runs rich. Starts and runs fine. Shifts good most of time. Wonder if step down transformer in PCM is bad? Do not know what to do to fix. Code 12 says battery has been disconnected when it has not been.

Running rich (high hydrocarbons HC) indicates either a bad catalytic converter, leaking injectors, bad plugs and/or wires, or a bad MAP sensor. A rich condition over time will slowly burn up a cat, so you may have more than one problem leading to the rich exhaust. Look at these places first before condemning the PCM.

Code 12 comes from disconnecting the battery within the last 50 key starts and is unrelated to your rich condition.

Start here and see what happens.