The transmission is going into "limp" mode which stays in second gear to allow you to drive slowly to the repair shop with needing a tow truck. You have to turn the ignition switch off and restart the engine to get out of limp mode. It will continue to shift fine until the problem is detected again.
As a general rule, if you start the engine, shift to drive, and it goes to second gear right away and stays there, it is a sensor-related problem or something else electrical. If it defaults to limp mode immediately during or right after an up-shift, especially under relatively light throttle, it is most likely slippage in one of the clutch packs. There's an input speed sensor that monitors engine rpm, and an output speed sensor that monitors axle shaft speed. The Transmission Computer knows how fast the output speed sensor should be going according to which gear it's in. If those two don't agree, the computer knows slippage is taking place.
The place to start is by having the diagnostic fault codes read in the Transmission Computer. Turning off the Check Engine light just erases any codes stored in the Engine Computer. The purpose of that light is to tell you to READ those codes, not erase them. Even if they erased the codes to turn the light off, the transmission codes should still be stored in the computer unless they erased them too. Once the problem occurred again, those codes should be set in memory again. Have them read at a transmission specialty shop.
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Thursday, July 7th, 2011 AT 7:33 PM