DIAGNOSIS The Homelink/Gatekeeper system in your car needs to be synced using the 'learn button' on the opener motor unit, not just the remote programming steps.
FIX Use the motor unit's learn button (usually yellow or purple).
1. In your garage, locate the learn button on the back or side of the motor unit. Press and release it. The indicator light will illuminate or blink.
2. You have 30 seconds. Go to your car and press and hold the desired Homelink button. Hold it until the garage door opener's lights flash or you hear a click (this can take up to 30 seconds).
3. Test the button from your car. You may need to press it twice—first to 'wake' the system, second to operate the door.
IF THAT DIDN'T WORK Some older car Homelink systems require an additional step with a handheld remote. Have a working LiftMaster remote in hand, hold it near the car's rearview mirror, and press both the remote button and the car's Homelink button simultaneously during the learn mode window.
HEADS UP The opener's learn mode erases all previously programmed remotes and keypads if you hold the learn button down too long (usually >10 seconds). A quick press only adds a new device.