When testing a garage door opener, what should be placed under the door?

When testing a garage door opener, placing a 2x4 laid flat under the door is the most effective choice. The purpose of using the 2x4 is to ensure that the door's safety reverse mechanism functions correctly. This mechanism is designed to prevent injury or damage by reversing the door's motion if it encounters an obstacle.

The flat 2x4 provides a solid, stable surface that simulates the type of obstruction that a garage door may encounter in real-world scenarios. When the door comes down and makes contact with the 2x4, the opener should detect the resistance and cause the door to reverse its direction, demonstrating that the safety features are operational.

Using other options such as a wooden plank, rubber mat, or cardboard box may not provide the same level of stability or consistency in testing the opener’s safety mechanisms. A 2x4 is typically more effective in ensuring that the safety reversal is tested accurately because it consistently resists the door's pressure, mimicking a real obstruction.