P0123 Throttle position sensor circuit high input
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ò Open or short to battery between Throttle Position Sensor (TPS) C144-3 and (ECM) C211-73.
ò Open between (TPS) C144-1 and (ECM) C211-71.
ò Faulty TPS.

OK. Kia is not clear about the consequences of that code on general operation but on my other car, a throttle position sensor error code inhibits 4th gear, TCC and cruise control...so this might look like a transmisison trouble (does not go past 3rd gear) but it is not. I cannot say if KIA has programmed the same but I would surely investigate, repair and clear the TPS code before anything else. Keep in mind that sensor connectors are a common reported cause of problems .