Apollo 16 went into lunar orbit on April 19, 1972. The next day, the Service Module engine was used to lower the orbit to 20 km (12.4 miles). After undocking, it was normal for the astronaut in the Command Module to fire the Service Module engine to raise the orbit back to approximately 110 km (68 miles). When Mattingly did this, he noticed a malfunction in the controller for the engine so he did not fire the engine. If there was something wrong with this engine, the Command Service Module and Lunar Module might have to dock and the astronauts would have to use the Lunar Module engine to get home. After 6 hours of discussion, Mission Control determined that the engine was safe to use and the astronauts were given the go ahead to land.

Apollo 16 Lunar Module at undocking.