Have 2016 GM 2500 high country 6.6 diesel.

60k miles. Don't drive it often. But roads were bad other day and decided to drive it to work. About Mile from house truck hesitated and check engine light came on. Upon getting to first stop and taking out the truck felt like it was in higher gear.

Was like this all the way to work. Got there let truck sit. Went out with it cold and it shifted normal until it went few miles then it acted the same.

Drive back to work and upon leaving it was the same.. Worked then stopped.

Took it to buddies with snap on. Had bunch of codes. Cleared them and it ran flawless. Drove it for about 20 miles. Bought him case of beer. Dropped it back off and it did it again about 1/2 mile from home. Took it back and this time besides generic codes it showed. P0721 which is trans output sensor.

Crawled under it checked connection, as well as TCM connections. Nothing apparent. No change.

Next day removed sensor and took it up auto parts that supposedly had one. Wrong connector type. Had to order one.

Cleaned it off nothing below o ring. Reinstalled and truck is way worse. Can't get it out of first and wouldn't go in reverse.

Hoping I'm on the right track. New sensor should be here today.

Any techs in here that can provide other checks as I'm never this lucky.

Pins in TCM all look and feel tight. Likewise on output sensor.

Thanks

When you were looking at tcm connections, because it is located on radiator shroud, did you look at the harness to see whether it was contacting accessory drive belt. I fixed a few of these years ago. There is a service bulletin about it

Sorry tcm is up by brake booster that year but harness contact is still at drive belt

Make sure to put alison or GM parts back in it. If that doesnt fix it, then take to dealer and have them re flash TCM.

quote:

Originally posted by Mark8215:
Sorry tcm is up by brake booster that year but harness contact is still at drive belt

It is on the fan shroud. Sensor they ordered was wrong connector.

If the sensor does not resolve your issue, I would place blame on the TCM. My Dmax without notice did a hard downshift when coming to a stop one day and went into limp mode. Don't recall the codes, however it had 4 or 5 that directed me to valve body issues. Cleared the codes and all the same codes reappeared on the next drive after a 20 mile trip. After replacing suspected solenoid(s) and inspecting all the wiring and connectors the same thing continued. Installed a new TCM and all is well... Good Luck

Thanks for comments. It was the output sensor. Easy fix, harder to find part numbers for it. No auto parts stores had it. GM dealer couldn't even find it in their system.

There were two connector types used during model year.

I researched it and got it off Amazon.