To reset the tire pressure monitoring system (TPMS) on a 2021 BMW 3 Series, follow these detailed steps:

1) Check the Tire Pressure Manually

• Before resetting the TPMS, it's essential to ensure all tires are properly inflated according to the manufacturer's specifications, which can be found in the owner's manual or on the placard inside the driver’s side door jamb.
• Use a reliable tire pressure gauge to check the tire pressures, and adjust as necessary.

2) Start Your BMW and Access the iDrive System

• Power on your BMW by pressing the "Start" button without pressing the brake pedal to ensure it’s in "Accessory" mode.
• Navigate to the main menu of your iDrive system. This is typically accessed by pressing the control knob on the center console or the main menu button adjacent to it.

3) Select 'Vehicle Status' from the iDrive Menu

• Within the iDrive menu, look for the 'Vehicle Apps' or directly 'Vehicle Status' and select it using the control knob.
• This menu contains information about various diagnostic and status reports for your vehicle.

4) Choose 'TPM (Tire Pressure Monitor)'

• Once you're in the Vehicle Status menu, scroll to find 'Tire Pressure Monitor'.
• Select this option to access the settings related to tire pressure.

5) Start the Reset Procedure

• Within the Tire Pressure Monitor menu, you will see an option to 'Perform reset' or a similar choice depending on BMW’s specific software updates.
• Choose 'Perform reset' and confirm your selection. The system will prompt you to drive the car to allow the reset process to complete.

6) Drive Your Vehicle

• For the TPMS reset to be fully completed, drive your BMW at a speed above 20 mph (about 32 km/h) for approximately 10 to 15 minutes.
• The system will calibrate and memorize the new tire pressures during this driving period.

7) Verify the Reset

• After driving, check the iDrive display to ensure the reset was successful. The TPMS alert should not be illuminated if the system has recognized the tire pressures as being within the specified range.
• If the light remains on, repeat the check of tire pressures and the reset procedure.

These steps should enable you to reset the tire pressure monitoring system on your 2021 BMW 3 Series effectively. If the issue persists, there could be a sensor malfunction that requires professional attention at a BMW service center.