How to Disable Reverse Beep in a Toyota Prius
The right way to disable the reverse beep in a Toyota Prius depends on what year the car was made. In many newer models, you'll need to access the on board computer using an application on your phone and OBDII Bluetooth adapter. Find the OBDII port in your car, which is typically under the dashboard, insert the adapter into the port, and download a code application for your phone. Once your phone and adapter are synced, locate the setting for the reverse beep and change it to “off” or “beep once.” Alternatively, some models will have you download a coding application for your phone that you can connect to the Bluetooth adapter. Once you enter the correct string of codes, you can access your car's computer display and change the backup horn settings.
Start the car. Prius cars made from 2004 to 2009, called "Generation 2," have a combination of in-car commands you can utilize to disable the reverse beeping, but the vehicle must be started in order to do so.  Start a Toyota Prius by inserting the key into the ignition. Unlike a normal vehicle, you will not need to turn the key. Instead, press your foot firmly on the brake and hit the start button to the driver's right. The Prius is extremely quiet, so pay attention to the instrumentation to know if it is running. Some newer model cars will not require that you insert the key, as long as it's in the vehicle. Press the Trip button until ODO shows then turn the car off. On the right side of the steering wheel, there are a number of buttons you can use to control the settings and display of the dashboard. Press the button labeled “TRIP” until the letters “ODO” appear on the display. On some models of Prius, the button may be next to the odometer on the dashboard itself. In later generation models of the Prius, this function will not display ODO and this method will not work. You may need to press the button more than once to make it display the letters “ODO.” Turn the vehicle off the letters “ODO” appear. Start the car again. Wait a second, then restart the car in the same manner as before. Turning the vehicle off and then on again is an important part of the series of commands that indicates to the computer that you wish to turn the backup alarm off. Be sure the engine is off before attempting to start it again. Press and hold the TRIP button for 10 seconds. With the vehicle running once again, hold down the same “TRIP” button for a full ten seconds or longer. The display may change to reflect that you have held the button down on some cars, but it may not always. Count to ten to yourself before letting go of the button. If the display changes you can let go. Step on the brake and shift into reverse. Shift the vehicle into reverse by pulling the shift lever backward out of park. Remember to keep your foot planted firmly on the brake while shifting out of park to prevent the car from moving. The reverse beep may still sound at this point. Shift back into park. After you feel the transmission shift into reverse, press forward again on the shift lever to put the vehicle back into park. This is the final step in the sequence to change the reverse beep settings in the car's computer. Turn the car off again. The next time you start the car, the reverse beeping will be disabled. Re-engage the reverse beep. If you decide that you would like to turn the reverse beep back on in your Prius, you will need to repeat the process again in the exact same manner. Going through the steps each time serves to engage or disengage the reverse alarm, so completing each of these steps in the same order will reactive that alarm. If you have difficulty repeating the process, take the vehicle to your local Toyota dealership and they can reactivate the reverse alarm for you. Purchase the adapter. In many newer models of the Prius, you cannot disable the reverse beep using the method listed above and must instead access the on board computer using an application on your phone and Bluetooth adapter. Purchase an OBDII Bluetooth adapter from an automotive or electronics specialty store. Some Bluetooth OBDII adapters are designed specifically for certain smart phones, so be sure to purchase one that works with yours. Android phones may be better for this specific application. Locate the OBDII port in your car. The OBDII port in your vehicle is located on the underside of the dashboard on the driver's side.  It may be difficult to spot, as it tends to be made of black plastic. There are multiple metal pins inside the port and there should be no others like it in the area. If you are having difficulty locating the OBDII port in your Prius, refer to the owner's manual to assist you in locating it. Insert the adapter into the port. Insert the new OBDII Bluetooth adapter into the OBDII port in your Prius. Because the adapter is wireless, it should not hang down very far. Some adapters will light up when plugged into a vehicle with the power on. Make sure the adapter is seated firmly in the port so it has a strong connection. Press the start button without your foot on the brake to engage the car's electrical system. Download a code application for your phone. There are a number of applications available for smart phones that can be used to read or adjust the settings in your vehicle when combined with an OBDII Bluetooth adapter. Carista, OBD Fusion and DashCommand are each options that may work for your specific smart phone. Open the application and create an account if need be. Turn on Bluetooth on your smart phone. Tie the phone to the adapter. With the application of your choice running and Bluetooth enabled, the phone should automatically sync with the adapter. If it does not, try turning Bluetooth off and then on again. Ensure the vehicle has the power running, but not the engine. You may need to open settings and tell the phone to sync with the adapter. If you have a Bluetooth password on your phone, you will need to enter it for the two to connect. Locate the setting for the reverse beep. Depending on the application and phone you are using, there will be a number of settings available that you can modify directly from your phone. One of these settings is for the reverse beep. Scroll through your options until you come across it. Some phones may be unable to access this menu. If that is the case, you will have to edit the code yourself. Change the setting to “off” or “beep once. ” If the setting appears as an option in your phone, you will be able to change the options from “on” to either “off” or “beep once.” If you choose the “beep once” setting, the vehicle will honk the horn when put into reverse, but that's all. You can go back in and change this setting again any time you want. After changing the setting, close the application and remove the adaptor from the OBDII port. If you ever decide to turn the reverse alarm back on, simply change the setting back to "on." Download a coding application for your phone. In much the same way you can make changes to the settings in your car's computer using applications, there are others that are designed to allow you to manipulate the settings using programming code. The best application for this is called ELM327 app for Android or Apple devices. There may be other options available to you for download. Ensure the application you choose allows you to manually input lines of code to interact with the car's computer. Connect the application to the Bluetooth adaptor. Insert the OBDII Bluetooth adaptor into the OBDII port and hit the “Start” button on the Prius without pressing the brake pedal to the floor. This will engage the vehicle's electrical system. Be aware, modifying the code in your vehicle's computer may cause other issues and should only be done by professionals. Doing this may void your warranty if you have one. Enter the appropriate string of codes. Once the application is connected to the Bluetooth adaptor, type in the series of codes required to adjust the settings on the reverse alarm. These codes serve as prompts, and the vehicle will respond with codes as well. Type the following codes into the application:  Enter the command “AT SH 7c0” and the vehicle should respond, “ok.” Type “21ac” and the response should be “61 AC 00” which represents the reverse horn setting. Enter the code “3bac40” which will adjust the setting. You will receive two lines of code from the car indicating that you are done. Complete the coding by typing “21ac” again. When you type the command “21ac” again, it will prompt the computer to display the current backup horn setting. While “00” represented having the setting on, it should now read “40” which indicates that the horn will only beep once.  Close the application once you have verified the new setting is in place. Start the vehicle and test to ensure the backup horn is off. Take the Prius to a dealership. If the horn still beeps while the vehicle is in reverse, take it to your local Toyota Dealership. They have the tools on hand to deactivate the reverse beep without voiding your warranty or causing any further issues. The price for this service may range from $50 to $200 depending on the availability of the tools needed and the dealership itself. This is the safest way to deactivate the backup horn. Turn the reverse alarm back on. If you decide you'd like to turn the reverse alarm back on at any point, reconnect to the Bluetooth adapter and enter the same series of codes to adjust it back to its original setting. Enter the command “AT SH 7c0” and the vehicle should respond, “ok.” Type “21ac” and the response should be “61 AC 40” which represents the reverse alarm being set to off. Enter the code “3bac00” which will re-activate the alarm. You will receive two lines of code from the car indicating that you are done. 
Start the car. Press the Trip button until ODO shows then turn the car off. Start the car again. Press and hold the TRIP button for 10 seconds. Step on the brake and shift into reverse. Shift back into park. Re-engage the reverse beep. Purchase the adapter. Locate the OBDII port in your car. Insert the adapter into the port. Download a code application for your phone. Tie the phone to the adapter. Locate the setting for the reverse beep. Change the setting to “off” or “beep once. Download a coding application for your phone. Connect the application to the Bluetooth adaptor. Enter the appropriate string of codes. Complete the coding by typing “21ac” again. Take the Prius to a dealership. Turn the reverse alarm back on. 