Code issues after transmission replacement? The P1603 and U0101 codes?

What codes are they? It sounds like the fluid is low or you have an input shaft speed sensor that is not working.

here are two guides that can help with the location of the sensor in the images below:

https://www.2carpros.com/articles/how-to-check-and-add-transmission-fluid

and

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Check out the images (below). Let us know what happens and please upload pictures or videos of the problem so we can see what's going on.

Here is information on the codes:

The P1603 and U0101 codes are both related to the powertrain and network communication systems in a vehicle. Let's take a closer look at each code:

P1603 - Powertrain

The P1603 code is a generic powertrain code that indicates a malfunction in the internal control module memory. It suggests that there may be an issue with the Engine Control Module (ECM) or the programming within it. Some possible causes for this code include:
Faulty ECM: There may be an internal fault within the ECM itself, such as a damaged circuit or component.

ECM reprogramming: If the ECM has been recently reprogrammed or updated, it's possible that the programming was not performed correctly.

Low battery voltage: Insufficient voltage from the battery or charging system can cause errors in the ECM operation.

U0101 - Network Codes
The U0101 code refers to a communication problem within the network of the vehicle. It specifically indicates a loss of communication between the ECM and the Transmission Control Module (TCM) or other control modules. Some possible causes for this code include:

Faulty wiring or connectors: Damaged or loose wiring connections or corroded connectors can disrupt the communication between modules.

Faulty ECM or TCM: A malfunction within the ECM or TCM can lead to a loss of communication.

Network bus issues: Problems with the network bus, such as a short or open circuit, can prevent communication between modules.

So it seems like one of the main grounds was left off when the transmission works was done please double check the grounds around the transmission.

I am glad you are feeling better, back injuries are rough to say the least. Please let us know how it goes.