Shifting issue. 04 avalanche Z71

[Sstevieboy0208]

So I have less that 200K and yesterday while on the interstate I pushed the gas and the truck shifted down or in overdrive to pass someone then acted like it was stuck in that gear so I pulled over put it in park then back to drive and it went fine no issues drive it about 150 or 200 miles no issues. Then it did it again today and also back to normal like nothing happened. Any advice will help. The truck shifts just fine very smooth and the only code i have is bank 1 downstream o2 sensor.

 

[EXT4ME]

At 200K, if that 2004 hasn't had it's transmission rebuilt yet, I bet it's time.

While my 2003 Z-66 with 251K miles on it is still running with the OEM build on the transmission, my 2004 EXT with 236K had it's first rebuild at 125K.

I have had every transmission in every GM truck I have owned in the past 40 years of driving, rebuilt during the time I owned those trucks.

And I do not abuse my vehicles.

Most between 80K and 125K miles.

I'm just waiting for the 2003 Z-66 to crap out.

I know it's coming some day.