Hello all....great forum here. I've been searching around for possible solutions to a problem that my son's car started exhibiting yesterday. He called me to tell me that it wasn't shifting up when he had it in D4, but found that it would work if he left it in D3 (obviously not shifting up into overdrive though). Today he called to let me know it won't go beyond 2nd gear, so this obviously makes it pretty UN-driveable. I've done some searching on here and the two most common suggestions that pop up are the TCU (Transmission Control Unit) having some cooked capacitors (and resistors?) or the Transmission solenoids.
My kid's cars have been KILLING me with repair bills lately, so I'm really hoping this is something I may be able to repair on my own. Maybe not, but at least I'd like some ideas of what may be causing this. Any help would be GREATLY appreciated.

Thanks in advance guys,

Warner