Code P0700 2006.5 kia optima 2.4L, transmission gets stuck in 3rd gear. Calling for code P0700. I can't get a mechanic to tell me anything but replacing the transmission and I think it could be just a sensor from other forums I've read. This started a couple years ago, I drove it out since I didn't have funds to replace the transmission back then. So March 2011 my engine blew, after replacing the engine, the transmission problem just went away. Til October 2011. So I don't understand why it stopped and came back again. I had a transmission shop replace the solenoid it was calling for 2 years ago and that didn't do anything.

P0700 = TCU request for 'MIL' on (IE transmission control unit has detected a fault and wants you to know about it.)

Reasons:

A communication line exists between the Engine Control Module(ECM) and the Trans-axle Control Module (TCM). The sole purpose of this communication line is for the TCM to notify the ECM to activate the Malfunction Indicator Lamp (MIL) when a serious trans-axle failure has occurred.

DTC Description
If the signal "MIL ON" is requested from TCU or it is no signal from TCU, the ECM judged as a fault and DTC is set.

Kia's first suggestion is to check for damaged wiring or connectors.

Try this: Disconnect your battery for about 5 minutes to clear the fault. Then take it for a spin and see how it shifts... good luck.