The immobilizer light comes on when the key is inserted, turns off for a second, then comes back on and stays solid red.

My car all of the sudden won't start. no Crank, nothing. The lights however all come on, battery is good, alternator is good. I have tried removing the battery terminals for 3 hours, I've tried leaving the key in the ignition for hours, none of this has worked.

Could this be the PCM or is this an immobilizer only problem?

 

Probably more of a BCM, but the thing is usually you have to replace the PCM as well or it relearns the problem.