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This code sets when the overdrive clutch is commanded on but there is no RPM decrease. This can be caused by the solenoid pack mounted to the valve body, a leaking overdrive clutch piston, slipping overdrive clutch, or geartrain damage in the overdrive unit.
The first thing I would do if I was diagnosing it would be to use a pressure gauge to see if the overdrive clutch was being applied when it should be. If the pressure was correct then the overdrive unit would need to come off the back of the trans, if there was no apply pressure being sent when it should be shifting into overdrive then you would know some electrical diagnosis would need to be done, and if ok then the valve body would have to come out to inspect/replace the solenoid pack. You are most likely either going to need a solenoid pack or there is damage in the overdrive unit.
You're welcome, and good luck!