Looked the manual over and there is not a lot to check. The code is for that solenoid either stuck on or off, the valve that solenoid controls is blocked or stuck, or a malfunction of the lockup clutch. There is a yellow wire with a black tracer that goes from the ECM to the solenoid. It is supposed to supply battery voltage (9 to 14V) at between 47-52 MPH. If you pull the ECM connector off the ECM, looking at the ECM there will be 4 sets of plugs. Right to left they are E4,E5,E6,E7. E7 has 10 pins on the top row numbered 1-10 right to left. 4th row down, far right is pin 27 which should have had that yellow/black wire going to it from the plug. Between that pin and ground (they use E5 pin 16, bottom row of th E5 plug, far left as ground but you can use chassis ground) should be 11-15 ohms. That is the only check of the ECM. You should have some continuity through the soleniod coil if you test between the Yellow/black wire and ground. If all that looks good you may have to find a way to check the voltage on that Y/Bk wire from the ECM while driving to see if it supplies the lockup voltage. It won't lockup if the trans temp says it is not warm enough or if the speed sensors have problems, but this should isolate the solenoid problem if there is one.