It is definitely because you have no sensors wired in. They complete the circuit. And I 3rd installing the sensors even if it costs you money (the opener was already free anyway). They are a safety item installed for a reason. I remember reading about a guy who's door was giving him problems and he wired past the sensors and his 3 year old walked under it when it was shutting. Killed him. Can you imagine what that guy felt like?
Come to think of it I had some of those sensors out in the garage at one time. I'll look tomorrow and let you know if I still have them.
