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The U0404 sets in the trans controller when it sees implausible data from the electronic shifter module (ESM). This module is on top of the shifter assembly, actually part of the upper shifter bezel. I've seen a few issues with these, it's very likely that this module will need to be replaced.
This should be a separate issue from the park assist problem. To start diagnosis of the park assist problem you would have to start by having the codes read from the park assist module. This will require a dealer scan tool, possibly a high end aftermarket scan tool may be able to access this module. Most likely one of the rear park assist sensors has failed and will need to be replaced.