Why is my garage door opening when i start my car

Your car must be emitting the right signal frequency to operate the garage door opener. You'll have to change the frequency in the door opener to something different. Look in the manual for the garage door opener to see how to do this.

Rowefast is right if your door opener is quite old. But this car may be so advanced that it opens the door automatically when started, using the remote door code that was programmed in. Newer door openers not only have to match the frequency but I think there is a rolling code that has to match. Try this as a test.Turn the motor off a minute, then restart it to see if the second signal will close the door. This won't prove the source of the unknown signal, only that it happens with each motor startup. Also, check overhead to see if any flying saucers are messing with you. I hope this helps.