First off I want to say I tried searching for already existing posts, couldn't find any.

With that being said, I would like some help with a strange problem I had the other day. I have a 2003 automatic with about 135k on it. I have no internal mods or anything, just a plenum spacer and silicon intake tube basically. The other day I went to work, no issues at all, when I was coming home however I noticed very quickly that after 3rd or 4th(I think it was 4th) gear I lost basically all my power. I could push the pedal to the floor and get absolutely nothin, but my speed would still rise, albeit very slowly. In addition the rpms would continue to rise well over 3k without entering the next gear. After I got home I checked my trans fluid, was perfectly fine, good level, not burnt at all. Nothing else about the car was running strangely.

I had a friend who suggested I try the manual mode to see if it was any different. The next morning I went out for a drive and did just that. Everything was back to normal, regular power and acceleration, gears shifting just fine. I then go back to auto and the same thing. Everything was working as usual again. My check engine light never came on and to my knowledge I have no codes.

Is a sensor dying somewhere or something?

 

I'm no automatic expert, but it's very possible something inside the trans is acting up. If you're confident the transmission has been properly maintained/serviced and the issue is intermittent, I would guess it's an electrical component. I'm thinking a solenoid is starting to fail. Usually when these start acting up, code(s) will be thrown. I found this old post, the OP appears to be complaining about the exact same issue (Stuck in 4th gear, little to no power).

Even though a check engine light has not been set yet, I would highly recommend having a full scan run, with focus on the TCU. It's possible there is/are pending code(s) stored in memory. If you can't find any codes and the issue cannot be reproduced, then you're left with just playing roulette. Keep driving the Z until the issue becomes more common or start dropping cash on parts that "might" cure the problem. Personally I don't like guessing on fixes with my money, I want to see proper diagnosis/testing to ensure a proper fix. I don't know if you have experienced transmission shops in your area, but that's where I would start looking for professional advice. Most dealerships and private autoshops don't touch automatic transmissions due to their complexity making it a specialized profession in it's own right. If you go to a dealer, they will most likely just recommend replacing the entire unit or outsource the repair to a trans shop.
Good Luck!
-Icer