Code P0621 - Grand Am Pontiac Cars & Trucks

This is not that clean cut, ECM failure is not that common. The battery died and that caused the no start. This could also account for the setting of that code because of loss of field charge in the alternator. A dealership can write off parts that are not bad, if they did not do the work, you are stuck with the parts. ECM is not cheap. I would check the CPS first.

Without testing we'd only be guessing ! You have no idea of the complexity of the electronics on your vehicle . Replacing the ECM would mean a trip back to the dealer to have it programmed . Without programming your vehicle will never start . You had new battery an alternator installed , did it start an run when you picked it up. The charging system on your vehicle is computer controlled . DTC P0621
DTC P0621 : Generator L-Terminal Circuit
The engine control module (ECM) uses the generator turn ON signal circuit to control the load of the generator on the engine. A high side driver in the ECM applies a voltage to the voltage regulator. This signals the voltage regulator to turn the field circuit ON and OFF. The ECM monitors the state of the generator turn ON signal circuit. The ECM should detect low voltage on generator turn on signal circuit when the ignition is ON and the engine is OFF, or when the charging system malfunctions. With the engine running, the ECM should detect high voltage on the generator turn on signal circuit. The ECM performs key ON and RUN tests to determine the status of the generator turn on signal circuit.

Your best bet take it back to the dealer .

Hey Joe , The first thing to do would be check for battery voltage on the heavier gage wire on the back of the alternator , if there is no voltage here there is a open circuit ! Which means the fusible link fried . This wire comes from the starter ! This charging system is controlled by the PCM , but they are rarely the problem . Do you have the charge indicator lit on the dash ? Do you have a check engine light on ? This system can set DTC'S - diagnostic trouble codes !
DTC B1328 DTC P0562 DTC P0563 DTC P0621 DTC P0622
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I had the exact same problem last year with my 2001 Alero 3.4L V6. It turned out to be that most of the copper strands in the multi-strand ground wire were broken, in the wiring harness passing over the transmission on the way to the fuel pump behind the fuel tank. The same ground wire is shared with several of the warning gages on the dash, causing several to remain lighted all the time. Both of the check engine lights, the brake light, the low oil light, and ABS light would all remain on all the time. Once the frayed portion of the ground wire above the transmission was repaired, the lights stayed out and the car started running normal, with the fuel now actually pumping gas under continuous high pressure to the engine.

have your alternator rechecked it could be bad even tho its is new this is very common .

Google------ the the OBD Codes you have & your vehicle
& read up

The codes are for two different circuits that involve the alternator and the computer. One is for the field circuit and the other is the lamp indicator or gauge circuit. The PCM controls the voltage regulator inside the alternator.
You would need to take it to a shop that has the books, or buy a used copy of the GM shop manual to get the test procedures for both codes.
The problem could be in the wiring harness, or the computer.

Your truck has an antitheft system called passlock. It runs thru you bcm. When this system fails, your truck will start for 1-5 seconds and then stall. The fuel enable is switched off, and injectors fail to squirt. This may be your problem and is a "bypassable" problem. This link explains.

http://vatspasslockpasskeysecurityhelp.yolasite.com/

Sounds like dead battery cell Have bat checked.It's not a good idea to disconnect the battery on a modern computer controled vehicle while the car is running.

P0621 - indicates a failure in the alternator lamp curcuit.
P0304 - indicates a miss fire was detected in the #4 cylinder
U1040 - indicates a loss of communication with the ABS system
U1000 - Indicates a class 2 communication loss

None of those codes are relating to a transmission failure (P0700-P0702)

I would have a visual look at the transmission and verify that all electrical wires are connected and tight. Then I would have the transmission serviced if it hasn't been within the last 35,000 miles.

Have the trouble codes cleared and address the ones that return.

After that you may be looking at an internal transmission adjustment and/or repair.

Good luck.