I recently had my altenator replaced on my 1996 plymouth Neon.. After doing so, When I was driving about 10 minutes after the new alt. was in, my check engine light came on and now stays on. I replaced the Battery along with the altenator. Thinking that the fuel filter might be the problem I replaced that as well.. The Service Engine Soon light is still on. I did the onboard diagnostic thing, and got a code 12 : Direct battery input to PCM was disconnected within the last 50 key-on cycles
but I have NO idea what this means.. Can anyone help explain it?

I also got a code 21 : 02s sensor problem (oxygen sensor), upstream or down stream. I had a light on for my o2 sensor a long time ago and replcaed the O2 sensor and the light has never bothered be again, so I do not know why the error code gave me this also.

Can anyone tell me my Engline light problem? Thanks..

A code 12 is nothing, really. Just a bit of info on a harmless thing. If you got a code 21, that's why the CEL is on. A 12 wouldn't trip the CEL...

Have both your O2 sensors checked out. IIRC, voltage on them should read between 5-7 ohms. The upstream sensor is usually the first to go.

Thank you, I will try that out and hope I can finally get that Light to go away! I had replaced the O2censor a while back, I was not aware that there was another one.

Yeah you got your upstream in the base of the exhaust manifold, and you cant miss the downstream o2 sensor, just look under your car at you passenger side, its in plain view.:smile: