This was a test to launch a dummy Surveyor lunar lander spacecraft into a barycentric orbit towards a simulated Moon.
Unlike the two previous Surveyor mass model tests, this flight was supposed to demonstrate a restart capability for the Centaur upper stage. The Centaur-Surveyor combination successfully achieved parking orbit around Earth with a first firing, but when it came time for the second firing, the Centaur RL-10 engines fired for only a few seconds instead of 107 seconds.
A thrust imbalance left the payload tumbling and in an incorrect orbit of 113 x 208 miles (182 × 335 kilometers) at 30.7 degree inclination. The problem was later traced to a hydrogen peroxide leak in the ullage motors of the Centaur stage.
With no hope of reaching its ultimate orbit (planned for 104 x 236,000 miles or 167 × 380,000 kilometers), the payload reentered on May 5, 1966.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.