What problems could be causing the transmission to not shift past 2nd? Solenoids, TCC, etc? Any input will help. First I'm changing the Fluid and Filter because its dark brown (not gritty, or burnt smelling). Car has 163,000+ on it. The check engine light is on (like all GM cars do).

 

Find out why your check engine light is on and fix the problem(s). Then see if it shifts properly, replacing your dirty fluid certainly won't do any harm and might do some good.

The check engine lights on my GM cars with them fitted don't come on when I'm driving, if they did I would assume it was telling me something was wrong and I would investigate why.

Roger.

 

Yes, its probably a good thing to invest in a code reader. In the past the dealer would tell me 90% of the time it was an O2 sensor and reset it. Better to invest some money in a diagnostic tool than tear into a transmission and find nothing is wrong and it was a simple fuse or solenoid failing.