Code P0641, could it be a bad alternator?

Do you have any other codes? If the engine runs let's check the alternator output, this guide can help:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

If the alternator passes, and there are no other codes you may have a sensor that is shorted which is causing the 5-volt reference to be low. This will take some testing to figure out.

With the system on test the 5-volt references to each sensor and look for the one that is low and replace that sensor. This guide can help, and I have included the engine PCM wiring diagrams so you can see how the system works and which sensor wires to test.

https://www.2carpros.com/articles/how-to-check-wiring

The OBD-II trouble code P0641 in your 2011 Chevrolet Impala relates to the "Sensor Reference Voltage 'A' Circuit/Open." This code is often associated with an issue in the 5-volt reference circuit, which provides a stable voltage to various sensors and components in the engine management system. Here's what you need to know about this code:

Code Definition:
P0641 - Sensor Reference Voltage 'A' Circuit/Open

Possible Causes:

Faulty sensor (such as a throttle position sensor, manifold absolute pressure sensor, etc.) connected to the reference voltage circuit
Open circuit in the reference voltage circuit wiring
Poor electrical connection or corrosion at the sensor or harness connectors
Faulty Engine Control Module (ECM) or Powertrain Control Module (PCM)

Diagnostic Aids

* The 5 V reference 1 circuit provides 5 V to the following sensors:

- Manifold absolute pressure (MAP) sensor
- Fuel tank pressure (FTP) sensor
- Accelerator pedal position (APP) sensor 2
- Engine oil pressure (EOP) sensor
- Air conditioning (A/C) refrigerant pressure sensor
- Camshaft position sensor

* The 5 V reference 2 circuit provides 5 V to the following sensors:

- APP sensor 1
- Throttle position sensor 1 and 2
- Crankshaft position sensor

Check out the images (below). Please let us know how it goes.