I have a 1999 Ford E-150 4.6L V8 that I'm working on and I'm getting codes p0600 and p0203 and I'm wondering where I should start, the serial communication code or the fuel injector code? I've been chasing down this issue of it driving fine for about 3-4 minutes then it struggling after for about a month now and am getting ready to call it quits, but I'm trying to find any advice on testing or repairs I should try?

 

I would disconnect the battery long enough to reset the codes and recheck. Otherwise, I would look at wiring harnesses and connectors for that injector and the processors. The last thing I would suspect would be a computer problem as they are very reliable.

 

I did try unplugging the battery overnight to see if that helps, it's "driveable" but barely, the wires look good at the harnesses themselves but I haven't checked the PCM wiring harness yet

 

The P0600 code could be the PCM or it's ability to do what it supposed to do which could be connection related which could very possibly be a wire rubbing on a sharp object and working intermittently. To just replace parts is no guaranty you will hit the problem.
So the better way to go about this is to do a very thorough examination of where wiring harnesses contact sharp objects and actually look under the harness where the cut through the wire insulation might be. The harness involved would be the one that controls power and ground to the injectors.Not necessary to go on a wild goose chase looking at all harnesses. Just very carefully trace back from the individual injector wires that come from the ECM . To save some time you need only be concerned with the signal wire for cyl # 3. Once you know that wire color , it can save you some time . I suggest CAREFULLY because you could inadvertently move the wire without knowing where the problem was and it WILL come back.

It is no picnic to go swapping injectors, but it could come down to that. I would rather swap a coil assy first because that is much easier and can eliminate a possible coil problem.

 

I would check the all the grounds first before replacing the PCM.

Before pulling injectors, measure the resistance of the suspect injector and compare it to the others. Use the lowest range on your ohm meter. If there's an electrical issue with it, its resistance likely will differ from the rest of them.