next step is to remove that in line filter and see what happens. Unless it has to be there.
That was a pretty good filter when it was new and judging by the fact that it was restricted...it did its job....it probably had a magnet in it...too...
I would carefully cut it open and look to see what it has trapped...
And...I would get another one and put it in...made sure the flow is going the right way...convertor out to cooler is the front line...cooler to back of case is the return...
On the codes...as far as I can remember...the early two-digit codes are for simple parameters...probably out-of-range...like
@trackspeeder said...later styles of codes had circuit integrity (or not) and also performance codes...
I have a feeling that you are on the right track with thinking that the code popped up because of not being up to temp before the test...but I could be wrong...also...if the cooler filter was clogged...that would certainly cause the fluid to heat up more than normal...not enough flow out of the convertor...that could trigger an overheat limp mode...if there is such a thing on your system...
It is good that you followed the advice on the grounds...some people would shortcut that one...just look at them and go.."aww...looks ok..."
Grounds are my first project with anything...and hooking one up from the battery directly to the trans case...that was advised at some of the transmission classes I attended years ago...and it helped keep the pump bushing from flaking or spinning out of the pump body...
Cleaning out the main connector at the solenoid body..? I was going to suggest that...and the dielectric grease..? that too...
Good job...
Wishing you the best of luck with it...you are narrowing down the possibilities...