Guys, as someone who works in the industry, let me apologize for what you've gone through....
first... there's no such thing as an "undocumented code", period. if someone can say that with a straight face you need to get your car out of their shop immediately.
DTC's, ( "trouble codes" ) come in two flavors, universal, and manufacturer-specific.
codes that start with a "0" are universal, that is, they mean the same thing whether they pop up in a Buick, a Ford, or an Isuzu.
codes that start with a "1" are manufacturer, and sometimes model specific codes.
You can get all manufacturer specific codes at
www.obd-codes.com
If someone running a shop can't tell you what a code means, they certainly don't understand what a "trouble tree" is, and they're just going to toss parts at your car until you run out of money, or they get lucky.
There is NO code that says " if you get this code replace transmission".
The service manual for Isuzus advises technicians to narrow down a trans concern systematically -
first, Check the fluid level - the trans isn't dead until it's FULL and dead.
"metal in the pan" is a tactic that trans shops use to scare people into replacing GOOD transmissions. "Looks like a Chia Pet" leaves alot of leeway,
having some metal in the pan is normal - that's why there's a magnet ( why would you need one if there was never-ever-ever supposed to be metal there???)...
CHUNKS of metal in the pan is bad... if the guy is honest... or you watched him pull the pan... ( pretty easy to toss some shavings into a pan on the way up to tell you that you need to write the shop a check for a new bass boat, er... transmission...)
next, unplug the trans and shift manually through the gears - if the concern is still present, it's NOT wiring, or a computer, or a left rear seat heater module - it's in the trans.
The next step is a line pressure check.
Low line pressure = internal problem in trans = replace/rebuild it.
If a tech doesn't know what a line pressure check is, get your car out of his shop.
This doesn't take days of labor or cost thousands in parts. you're talking an hour or so. it's all in the service manual, it's not rocket science.
I really hope the OP is in California - they have this neat little organization called the "Bureau of Automotive Repair" - if a shop jerks you around like this in California you can report them to B.A.R. and watch them get their butts kicked into next week.
Good luck!