So this has been a bit of a ride, and I have gotten to the point I am totally stumped, but before I go to the mechanic with my tail between my legs, I thought maybe someone here could help me out. Car is an 02 Impala LS with the 3.8L

First, the catalytic converter plugged up. The car had been a bit down on power for a little while, but thought maybe just time for a tune-up, so I replaced all plugs/wires/coils, but did nothing to help. About a month later, going down the interstate, the car lost almost all power under acceleration. Trying to get up to speed on the on-ramp, anything more than about 10% throttle, car just stumbled and bucked, but could still get up to 65mph after a long distance, drove home about 200 miles, check engine light came on about half-way home, it was for the "EGR valve position" replaced EGR with Delco part, no improvement. Car sat, drove it to mechanic, now even more down on power, could only reach about 40, but made it there no odd readings etc. Mechanic told me cat is plugged, which I thought could be an issue, but quoted over $1,000 to fix it so told him to go to Hell and picked the car up. Half-way home it died, nothing dramatic, just quit running. Check engine light came on for "MAP absolute pressure reading too high, and random multiple misfire, towed it home, wont start and idle, but will start if I give it gas, sounds normal. Replaced the cat, both oxygen sensors, and the MAP, no improvement. Tried unplugging the MAF, car will start and idle at about 3500-4000rpm, didn't let it run for long, but no knocking smoking etc. engine sounds like it always did. Replaced MAF, same result.

I have been dealing with GM electro-bs for years now, but this I have no idea at this point. Any help would be appreciated.

Thank you.

Confusing.
Disconnect battery and clear all codes. What happens with a normal start? No throttle.

What carfixer007 said ^^^^^^^

. . . and sounds like IAC problem off the top of my head.

you cannot clear trouble code on any car newer than 96 by disconnecting the battery . the light might not be on after you disconnect, and reconnect, but the code is still present in the PCM . you can only clear by a scanner, or repairing the problem and then driving so many cycles without it seeing the problem anymore it will eventually clear.

If you clear it with a scanner will it still be in history? If so, isn't doing it by battery disconnect the same thing. If not, why not?

The only time you have a history codes is when the ECU detects a possible fault and it logs it as INACTIVE until X number of key cycles/drive cycles occurs, then the fault goes ACTIVE. If the fault does not go ACTIVE, then it stays in the history(length of time is unknown) All fault codes including pending/history are all deleted when the scantool tells the ECU to clear them out. Simply disconnecting the battery will not clear any fault codes on 96 and newer.

If you have rough running/hard start/misfire condition(when your check engine light is flashing at you, this means MULTIPLE CYLINDER MISFIRE and you should stop driving immediately to determine what the issue is) this could all be tied to your FUEL PRESSURE REGULATOR. When these fail, the intake manifold will suck raw fuel from the FPR and dump it into the intake causing excessive fuel to enter the cylinders causing misfires. There is a video on YT that talks about the 3800 FPR and a few threads here talk about the FPR with the YT video attached.