Hey there guys,
I have an 05 5A/T with 155k. This car has been great without any major issues. Yesterday on my way home the trans seemed to have jumped a gear and threw me into limp mode. The check engine light and "a/t check" light both turned on. I was able to drive it home, seems like limp mode keeps it in 4th gear.
Well I got home and put my scanner to it with the only code being P1762 which my scanner gives a generic "transmission system problem".
Well here is the kicker, if I clear the code and unhook my battery for 10 minutes, the car drives fine. It shifts into gear just like it used to and the problem seems to have gone away. Well after you put about 15 miles on it the trans again starts acting funny and the check engine light turns back on.
This leads me to believe its not a mechanical issue and a sensor is starting to malfunction.
I tried the battery trick a couple times and purposely beat on the car to try and force the problem to come up. Well that didn't' do anything, it only happens after you put some miles on it.
I tried searching this code and can't find a definitive answer, I'm pretty handy with a tool but when it comes to electrical/sensors I am lost. Is this an easy fix or something I'm going to have to take to a shop to? I've never had to take this car anywhere and I have stayed up on the maintenances myself.

The trans fluid has never been changed. Could that be the issue?

Thanks in advance guys.