2005 Chrysler 300 Do I need a pcm

Yup, that's what everyone starts with. The truth is Chrysler has extremely little trouble with their Engine Computers and that should be the last thing on your mind until everything else is ruled out.

The place to start is by reading and recording any diagnostic fault codes. Chrysler makes doing that yourself easier than any other manufacturer. Cycle the ignition switch from "off" to "run" three times within five seconds without cranking the engine, then watch the codes appear in the odometer display.

If there are no fault codes, probably the first suspect would be a plugged catalytic converter. The clue is you'll hear a smooth hiss from the tail pipe instead of the normal "putt putt".

The rough idle shouldn't be overlooked either. If that's due to a misfire, the spark plugs are the most likely cause. How long since they were replaced? Misfires will be detected by the Engine Computer. It will turn on the Check Engine light, and there will be a fault code indicating which cylinder is at fault.

Diagnostic fault codes never say to replace parts or that they're bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition.

I have heard of some running problems caused by a need to update the software in the Engine Computer, but those occurred at fairly low mileages and were expected to be handled under warranty. The computer doesn't have to be replaced for that.

Do you remember the exact fault code number?

If your mechanic believes a new computer is needed, the proof will be in the engine performance when the repairs are done. Too often I read that someone spent a pile of money on an incorrect diagnosis. That's when it's good to be at the dealership so they take the unneeded part out. When do-it-yourselfers buy the parts and install them themselves, those parts can't be returned, and that can get to be an expensive way to diagnose a problem.