Yet another Soviet attempt to soft-land a Ye-6 probe on the lunar surface ended in failure when the Blok L upper stage failed to fire for the trans-lunar injection burn. Instead, the spacecraft remained stranded in Earth orbit.
A later investigation indicated that there might have been a short circuit in the electric converter within the I-100 control system of the spacecraft (which also controlled the Blok L stage) preventing engine ignition. The spacecraft decayed five days later.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.