The resistance value of the solenoid changes with the sump oil temp.
My book says 3.26 ohms,+or- .26ohms at 68 degrees F. is the spec value of the solenoid.
Check all the basics first - connections at batteries ( power and ground), connections at ECU (power and ground too).
Disconnect trans harness connector from ECU to trans, clean,inspect,secure those connections.
Verify that they are good.
Check continuity on the A2 wire and the A31 wires.
A2 goes to pin A on trans
A31 goes to pin M on trans.
Maybe some corrosion, water, dirt, etc got in them. This is a very important first step in troubleshooting.
ECU is expensive $$$$$$$$$ ( about $2000)
I have found many problems with harnesses and connectors, only had one bad ECU.
Best of luck with your repair.