How to trace p0753 electrical issue with shift solenoid

What is your question?

Check engine light code P0753 tells me there is an issue with a shift solenoid A electrical. I am finding that when driving it will shift from 1st to 2nd, but no further even if go 40 to 70 miles an hour. How do I track down the open circuit or if it is a computer issue with the ECM or if the solenoid is bad?

What makes this problem better or worse?

Acceleration and driving over 20-25 mph

How long has your 2002 Toyota Highlander had this problem?

A few weeks