Trans overtemp

My appologies! The temp gauge that was peging out was the engine temp not trans. The trans overtemp message did however come on. I suppose that is just related to some type of overheat stratigy though. Anyway I monitered coolent temp and let the vehicle idle for approx. 20 min. Pressure built and temp increased to about 115 before the fan came on. About 125 the fan ramped up speeds but the vehicle did not seem to be cooling down. I shut it down at about 135. I could unplug the temp sensor and watch it default to -40 then go back to normal when pluged in. The t-stat is attached to the lower hose on this vehicle. When I felt the upper hose it was hot. The lower hose was cold closest to the radiator but as I felt it from the radiator to the engine it got progressively hotter. The coolent looked ok, wires and connectors looked to be ok, there didn't appear to be any exhaust/compression bubbling in the coolent when I was running it. All fluids look ok. Seems to me like it would either be a faulty t-stat or sensor. Again though, i'm a little suspect after all the work that was just done. Any suggestions before I start shotgunnin parts? Thanks again for your time and sorry for the mix up before.