4l60e transmission wont downshift back to first at stop unless you do it manually

Albert, Have truck scanned for trouble codes. To test a transmission solenoid, use hand held ohm meter. Instructions in second link below, the attached links are informative on the different procedures used to test solenoids.
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How to test shift solenoid 4L60E
http://etereman.com/blog/general-motors-transmission/how-to-diagnose-a-bad-solenoid-in-your-4l60e-transmission
Shift Solenoid and Tests GM 4 3L 5 0L 5 7L
How Can Tell If My 4L60 Transmission is BAD

What transmission does your trailblazer have ? automatic or manual ? If automatic 4L60E , it has no vacuum hose's ,unless you or someone else modified it . E in 4L60E stands for electronic . Back in the day there were vacuum hose's going to the automatic trans 1 to the vacuum modulator valve .

You are experiencing first stages of trans failure, and the 4l60 has a few

double post?

if it has an automatic OD then more than likely it has a lock up tourqe, google where the connection is and unplug the lock up selnoid and see if that helps or downshift manually and see what it does!

Time to take the truck to a transmission shop and get an estimate.

Best Manuals I honestly suggest you to not waste money on repairs. They're never satisfying and mechanics don't keep you in the loop very much. Try manuals and doing it yourself, you'll know better, and can also judge better.

How many miles? Have you maintained, serviced trans in past or just waited for this day to wonder what happened?

I believe you have a 4l60e transmission.Do you have a chck engine light? The unit may have a 3-2 downshift solenoid going bad or a possible band ''servo'' leaking. The band applies in 2nd and 4th.You may also have a shift solenoid starting to malfnction......Best

Take it to a transmission shop that has a Tech 2 scanner. The tech 2 scanner will not only scan the transmission but can also recalibrate the shift points. Other reason to why it wont shift under WOT it low transmission fluid pressure so check the fluid level and have a pressure test done on the line.
The 4L60E is a great transmission but there are some things that don't work well with them like some shift kits. The ECM is program the shift points which get's messed up if done wrong. best upgrade on the 4L60E was to swap out the servos for corvette servos or after market performance one's all for under $100.00 and in most case better then a after market shift kit.
Good luck and hope this help. take care not to burn up the transmission.