I have a 97 1500 with a 46re tranny. I was driving back to college which is about 180 miles from my home. Once I got back to school my check engine light came on and I got the p0743 code which is for the torque converter Lock up seloniod. I noticed no change in the way the truck drove. I changed the seloniod a few weeks later. The night I changed it, it drove for about 40 miles with no problems what so ever. Then the next day i drove it, no problem. It sat for off for 45 minutes and when I started it the check light came back on. The truck would not leave 3rd gear after that. I check the fluid and put more in it and it would shift correctly after that but the torque converter will not lock. I have no clue why. When changing the seloniod I noticed nothing alarming in the pan. It shifts just fine but will not go into lock up.

 

Check fluid level in neutral. You get an artificially high reading in park. (not that I think that is your problem...... perhaps a contributor.)

Check the 8 pin connector on the drivers side of the trans. Make sure it is clean, and tight, and that the wires aren't pulling out of the connector. Also check the connectors at the PCM.