Why does the open door warning beep go off when all the doors are closed in your 2003 Honda Odyssey?

Check your contacts on the inside of the door opening. The ones on the door look like thick pins and they make contact with metal cylanders on the crossbeam. If they don't make a good connection, you can get that error. Use either rubbing alcohol or an pencil eraser to clean the flat cylanders and your problem will be solved.