Transmission wont shift after replacing thermostat

I just purchased this Sant Fe, and it would not heat up to normal operating range. The check engine light came on, and trouble code referenced not operating to correct temp. I changed the thermostat, and this fixed the operating temperature issue. As soon as I changed it, I went on a test drive and was going approximately 60 mph when the vehicle shifted out of overdrive on its own. I slowed and stopped, turned around to head back towards home, and the vehicle will not come out of 3rd gear. Its an automatic transmission, and does not down shift, or shift into overdrive, remains in 3rd gear even if come to a stop and take off again. The trouble code now showing is p1529. After letting the vehicle sit overnight temps cool, (the vehicle never overheated) and cool to outside temps, I started the vehicle and went driving. Same thing, the vehicle shifts fine when cool, but once the engine reaches normal temp, shifts out of overdrive, into 3rd and will not shift out of 3rd.