trans code p0755

I have a 98 camry with code p0755 - shift solonoid valve#2 malfunction.-What are some of the most common causes
Any tips as to how to test it ,the issues that cause this and what to do also after replacing it if bad?
Also are these to be replaced in pairs with both 1st -2nd shift and 2nd to 3rd shift .

If i have high miles on it will adding the 2 -3 quarts of new fluid give me issues as to mixing with the old and part breaking up in transmission(ive heard a lot happens when u replace very old fluid with new or flush it fully)..will be replacing filter also but not the rest of fluid still in transmission at this time.
-Can you link a pic if possible .of the exact location on transmission inside pan .of it(looks similar to other shift solonoid near it )

Any additional info is appreciated.

I can't find P0755 in my Service Manual. Very strange. If you send my your e-mail address, I can send you a link to the service manual. There is transmission repair information.

There is a full code list here in the site on another post -Full List ,i dont know how to copy paste but it has all of them on it .
-http://www.toyotanation.com/forum/135-corolla-lounge/148676-obd-ii-trouble-codes-full-list.htmlm .

and yes --changing out the 2-3 rd shift solonoid fixed it