To replace the key fob battery in a 2019 BMW 3 Series, follow these detailed steps carefully:

Step 1: Gather Necessary Tools

• A small flathead screwdriver or a similar tool
• A new battery (CR2032)
• A clean cloth or paper towel

Step 2: Remove the Mechanical Key

• Locate the small button on the side of the key fob. This button is used to release the mechanical key.
• Press the button while simultaneously pulling out the mechanical key from the fob.
• Set the mechanical key aside for later use once the battery is replaced.

Step 3: Open the Key Fob Casing

• Identify the seam or groove along the key fob casing. This is where the two halves of the fob are joined.
• Using the flathead screwdriver, gently pry open the casing by inserting the tool into the seam. Twist gently until the casing begins to open.
• Continue to work your way around the seam until the casing can be separated completely. Be cautious not to damage the plastic.

Step 4: Remove the Old Battery

• Once the casing is open, locate the battery inside the key fob.
• Using the flathead screwdriver or just your fingers, gently lift and remove the old battery from its compartment.
• Dispose of the old battery properly, as it may be hazardous.

Step 5: Insert the New Battery

• Take the new CR2032 battery, ensuring it is the correct type for the fob.
• Insert the new battery into the compartment, making sure the positive (+) side is facing upwards. This is crucial for the battery to function correctly.

Step 6: Reassemble the Key Fob

• Align the two halves of the key fob casing together.
• Press down firmly until the casing clicks shut. Ensure that all edges are aligned and securely closed.
• Replace the mechanical key into the fob by sliding it back into its original position until it snaps into place.

Step 7: Test the Key Fob

• Test the key fob by pressing the buttons to ensure it is functioning correctly.
• If the fob doesn't respond, double-check that the battery is inserted correctly and that the casing is closed properly.

By following these steps, you should successfully replace the key fob battery in your 2019 BMW 3 Series.