I have a 2001 Suburban Z71 that won't shift into overdrive. It shifts thru the first 3 gears but not into overdrive. What could possibly be happening?

 

Could be the torque convertor lock solenoid which engages for overdrive.

 

thanks, I'll check it out!

 

How many miles on the tranny?

 

Usually when a no 3-4 shift occurs it is a faulty accumulator or servo, on occasion a stuck valve or bad pump can also cause the problem.
In any case it is not a home repair, however I always recommend the fluid and filter be changed before taking it in, it is cheap and sometimes solves the problem.

 

90,000 miles on the tranny... overdrive is working again... go figure!!! the Service Engine light went out... dealership "cannot duplicate problem". Kinda reminds me of going to the dentist with a toothache.