00 Cadillac SLS tranny codes and issue
 

I am working on a 00 Cadilac SLS, and the tranny shifts fine up until 4th gear will not go into 4th. I pulled codes P0741, P1639, P1845, P1860. I know what P0741 is (TCC Performance or Stuck Off) but not so much about the other codes or even what would cause them. Any ideas on the cause and the fix would be great.
Thanks
Joe

Pressure Control Solenoid would be my choice, controls line pressure in transmission.

00 Cadillac SLS tranny codes and issue
 

P1639 indicates a problem with the 5 volt reference circuit used for the A/C pressure sensor and fuel tank pressure sensor. It is probabably not related to your transmission problem, but will set the malfunction indicator lamp.

P1845 indicates a problem with the 2-3 shift solenoid valve control. You can verify that the problem is with the solenoid by testing it at the transmission side of the 20-way connector. Verify that there is a 20-40 ohm reading across terminals E and B. If there is a zero or infinite ohm reading the solenoid or wiring to the solenoid is bad. The solenoid is easy to change because it is located behind the transaxle oil pan. If the solenoid is OK look for a problem with the wiring harness or PCM.

P1845 is probably triggering the P0741 and P1860 codes because the PCM inhibits TCC engagent when it is set.