On my 03 the PCM I believe is in the center behind the dash against the firewall. I can get to the connectors from an opening in the trim on the drivers side under the dash.
I doubt that your computer is giving you an erroneous trouble code. There probably is some legitimate condition in it's self diagnostic that is not met and that is causing this code to appear.
I am not positive but I believe all the immobilzer function is contained in the PCM so unless there is some memory chip that you can transfer from your old PCM to your new one then you will need to get your ignition key programmed to the new PCM. I may be wrong on that.
You may want to look into the P0740 code first. A quick search for the problems that trigger this code are...
- Defective Torque Converter Lockup Solenoid
- Defective Engine Coolant Temperature Sensor
- Defective Torque Converter Clutch
- Defective Valve Body
- Dirty transmission fluid that restricts the hydraulic passages