2004 Astro, 2WD. Vehicle automatic shifts fine except does not shift 3rd->4th unless i play with the accelerator. Fluid looks perfect, color, smell and level are perfect. Shall I drop the pan and change 1 or more solenoids. No CEL.

 

How many miles on it?
Solenoids might be a waste of money if the trans is plum wore out.

 

210k miles. Does anyone have troubleshooting advice? Fluid is in excellent shape and has not had a recent fluid change. Can a solenoid cause just one shift to fail? Is that possible with 2 solenoids controlling 4 gears? Or would a problem with a solenoid prevent two shifts to fail? I only lost 4th. Anything else to check/replace with pan off? Willing to spend $20 on a solenoid, $30 on filter and pan gasket and maybe $35 on trans fluid. Waste 2 hours on my labor. But not if you are sure it can’t be the solenoid

 

A transmission shop would have to look at it and run some diagnostics to see. But he'll probably says the same thing : it's not worth putting money and time into a transmission with that many miles. Solenoid failures usually throw trouble codes.

 

As it happens, I am experiencing the same issue: difficult/inconsistent shifts to 4th gear (TC lockup).
2001 Astro, 80K miles, RWD

After weeks of head scratching and sleeping on it, I have decided that my issue is a restricted CatCon.
It's the original, as far as I can tell.

I am getting the same behavior. I get the P0101 DTC in strong headwinds and going uphill. And cannot hold speed on steeper hills.
As purchased, it had the CSFI (poppets) fuel injection spider, and I was having hard starts and poor mileage.

When I replaced it with the newer MPFI fuel injection spider, I found the intake was full of soot and grime. My TBI throttle plate's leading edge was COVERED in black crud! I believe unmetered fuel was making it run very rich, washing oil from the rings and dumping unburnt/incompletely burnt gas into the exhaust to be reinjected via the EGR valve.

I have ordered the new CatCon, which is a huge piece, includes Y Pipe and TWO converters in series.
I will return to say for certain if this was the root cause.