Need to replace the shift solenoid. where is it located?

The shift solenoid on a 2012 Chevrolet Impala is located inside the transmission. It is typically located near the top of the transmission, near the shift lever. In order to access the shift solenoid, you will need to lift the vehicle and remove the transmission pan. The shift solenoid is usually mounted on the side of the transmission and is connected to the transmission control module or the transmission valve body.

It is important to note that the location of the shift solenoid can vary depending on the specific model and year of your Chevrolet Impala. If you are not familiar with automotive repair and are unsure how to locate the shift solenoid, it is recommended that you seek the help of a professional mechanic.

The shift solenoid valves are two identical, electronic exhaust valves that control upshifts and downshifts in all forward gear ranges. These valves are normally open. These shift solenoid valves work together in a combination of ON and OFF sequences in order to control the positions of the 1-2, 2-3, and 3-4 shift valve trains. The TCM monitors numerous inputs to determine the appropriate solenoid state combination and transmission gear for the vehicle operating conditions.
Remove the transmission side cover. Refer to Control Valve Body Cover Replacement . Remove the 1-2 shift solenoid retainer clip (304), the 1-2 shift solenoid (305) with O-ring (303), the 1-2 shift valve (302), and the 1-2 shift valve spring (301).
Remove the PNP switch. Refer to Park/Neutral Position Switch Replacement . Remove the transmission upper side cover bolts. Install the engine support fixture. Refer to Engine Support Fixture in Engine Mechanical. Raise the vehicle. Refer to Lifting and Jacking the Vehicle in General Information. Remove the left front tire and wheel. Refer to Tire and Wheel Removal and Installation in Tires and Wheels. Remove the left inner fender liner. Refer to Front Fender Liner Replacement in Body Front End. Remove the wheel drive shaft from the transmission. Refer to Wheel Drive Shaft Replacement in Wheel Drive Shafts. Remove the power steering gear mounting bolts. Refer to Steering Gear Replacement in Power Steering. Remove the frame. Refer to Frame Replacement in Frame and Underbody.
This is a tough repair an needs special tools to do it . your best bet is take it to AAMCO

If your talking about the shift solenoids in the transmission, they are located on the valve body and will require removal of the transmission cover. If your talking about the BTSI (brake transmission shift interlock solenoid this is located on the shifter module.

I believe this vehicle is equipped with a shift lock that necessitates depressing the brake pedal before it can be shifted out of "Park". This is a safety feature designed into almost every car made nowadays to prevent unintended movement of the vehicle when shifting out of "Park". The Shift Lock solenoid is triggered by the brake light switch located near the top of the brake pedal arm. Starting with the "basics", check to see if the brake lights are working (they should illuminate when the brake pedal is pressed. If the brakes lights do not come on. then proceed to check the brake/stop light fuse. Replace if needed. Once the brake lights are functioning, the shift lock should "disengage" and allow you to shift out of "Park" when the brake pedal is pressed. If the fuses are okay, but still no brake lights, then the Brake Light Switch may need to be replaced.

Brake lights working but won't shift out of "Park"? Check for a separate fuse for the Shift Lock Solenoid. Replace if needed. Otherwise the Shift Lock Solenoid may need to be replaced. This will require disassembly of the console to access the part.

SHIFT SOLENOID INSIDE TRANSMISSION ON THE VALVE BODY.YOU NEED A REPAIR MANUAL WITH SAME TRANSMISSION IN VECHICLE TO LOCATE SHIFT SOLENOID SINCE YOU HAVE OTHER SHIFT SOLENOIDS.

Shift solenoids are in fact located inside transmissions. SHIFTER solenoids are located under the dash and are connected to the brake switch to enable you to change gearshift positions when the brake pedal is depressed.

What was the codes exactly that Auto Zone gave you.??? it might just be a simple wire harness that may have gone loose since you did some work on the speed sensor.
Here are the trouble codes for your car.  Good Luck and get the car scanned again for a more define information.

P0751 Shift Solenoid A Performance or Stuck Off

P0752 Shift Solenoid A Stuck On

P0753 Shift Solenoid A Electrical

P0754 Shift Solenoid A Intermittent

P0755 Shift Solenoid B Malfunction

P0756 Shift Solenoid B Performance or Stuck Off

P0757 Shift Solenoid B Stuck On

P0758 Shift Solenoid B Electrical

P0759 Shift Solenoid B Intermittent

P0760 Shift Solenoid C Malfunction

P0761 Shift Solenoid C Performance or Stuck Off

P0762 Shift Solenoid C Stuck On

P0763 Shift Solenoid C Electrical

P0764 Shift Solenoid C Intermittent

P0765 Shift Solenoid D Malfunction

P0766 Shift Solenoid D Performance or Stuck Off

P0767 Shift Solenoid D Stuck On

P0768 Shift Solenoid D Electrical

P0769 Shift Solenoid D Intermittent

P0770 Shift Solenoid E Malfunction

P0771 Shift Solenoid E Performance or Stuck Off

P0772 Shift Solenoid E Stuck On

P0773 Shift Solenoid E Electrical

P0774 Shift Solenoid E Intermittent

P0780 Shift Malfunction

Trans model is the 4L60E,and the shift solenoids are mounted on the valve body,just inside the bottom pan.There are 2 shift solenoids,one EPC solenoid (Electronic Pressure Control)One TCC PWM solenoid (Torque Convertor Clutch,Pulse Width Modulated),one 3-2 kickdown solenoid,and a manifold pressure switch.As far as a shift solenoid relay switch,i have never heard of that.The power for the transmission solenoids comes through the ignition switch ,and uses a fuse,not a relay.Accessing the solenoids only requires dropping the bottom pan,and the valve body does not have to be removed,to change the solenoids.The shift solenoids are the TWO solenoids right side by side and look identical..They are the only two solenoids inside the pan that are in fact identical.Good luck.

If you were to take the solenoid pack apart you would find a bad connection on that solenoid. You sill need a new pack installed.