Chosen Solution
My iPod recently started only playing audio out of the right channel of my headphones, for no apparent reason. Previously it had been known to not play audio at all, though I think that may have been due to cold when trying to use it, as the problem subsided. I purchased a new jack and hold button part here in the UK, as well as new screen (since that had lighting issues) and front panel and radial button, and have performed the replacement following the instructions on this site without any issues. However, the left channel still fails via the headphone jack. When playing back via a docking station, there is audio from both channels, which would lead me to suspect that the logic board is fine (though ISTBC). I have tested this both with Apple headphones and my own set which I know to work on both channels, and neither produce positive results.
I would then guess that the logic board of your iPod is unfortunately damaged in some way. I do not believe that the logic board has problems with processing audio, I believe it has a problem with outputting it to the headphone jack. The audio may come through the dock connector perfectly because it is a completely different port controlled by completely different parts. But, the board may have something going on when it tries to output audio to the headphones. Have you checked the connection of the headphone jack to the logic board? A partial disconnection can cause issues. If you go back in, you might consider taking a look around the headphone jack connector on the logic board. See if you notice any water damaged/charred parts. Hope that helps a little. Sorry it’s not a solution. :/ You’ve done a great job of ruling out the prime suspects though. If it wasn’t the headphone jack, it could have either been the headphones or the logic board. In this case, I suspect the logic board.
