P0700 Code Opinions Suggestions

diyer2 · August 27, 2017

2012 Sierra 5.3 6L80 trans. 4 wheel, 60k miles.

Trans shifts smooth in D, manual works smoothly, tow mode works as it should.

The first time it went into limp mode(about a year ago) was at start up. Started it up and backed out of a parking space, put it in D and could tell it wasn't right. Stopped, no CEL . Shut it off, restarted it and it was fine.

Fast forward to Friday. Start it up and it goes into limp mode, the check engine light comes on and it won't manually shift. Works in D.

We live in the mountains of Colorado so there are some hills and grades on the way home. At one point it wouldn't pull the grade so I stopped, put it in neutral then park. Put it in manual 1st then shift to second with no problems to get home. I run it in second manual to pull this grade on my way home every time.

Hooked up the lap top with Scantool.net and yes the code was P0700 and U0100. Cleared the codes and started it up. Sitting in the garage reverse and D works as does manual shift. Shifted through 1st to 6th sitting there.

Haven't needed it so it has been sitting. Waiting to see if it sets a code again right away or waits another year.

I did do a service recently but it has shifted normally and been flawless.

Maybe a TCM or shift solenoid glitch is what I was thinking. Wiring harness?

Any suggestions, opinions?