Check engine light comes on, the tachometer goes up to six, the gear won't shift, code P0846; transmission fluid pressure sensor. Is this costly to fix? Can any mechanic fix or does a transmission place have to do it? Any help/suggestions are much appreciated.

first check the tranny fluid with the engine warm and idling in park... i assume you have already done so... but i like to cover all the bases... if it is just a sensor you should be able to change it out yourself.