I'm working on a 1998 chevy cavalier. When its cold, it runs very rough, as it warms up it smooths out to just barely noticeable. It has new plugs, wires, coil, 2 new injectors, new fuel pressure regulator. The problem started after the alternator was replaced. A local shop did all the work but the fuel regulator. I found that to be leaking 3 minutes after first looking at the car. It has a P0200 open injector code, P0300 random miss fire code, and P0141 O2 sensor heater code. I have checked the injectors and wiring harness with a dvom for opens. All seems OK. I figure the miss fire code is set from the poor idle condition. I haven't went after the O2 sensor as of yet. The owner of this car has already spent over 800 with no results.

Cold idle problems have been linked to carbon build up on the back of intake valves.
Has anything been done to de-carbon the engine?
GM actually has a very good procedure to do a very good job of cleaning this up.

Before replacing the O2 sensor, check the fuse that's located inside of the driver's side front door; there's actually a separate fuse for the O2 heater.