Shuttle launch director Mike Leinbach stated that Endeavour’s launch will be no earlier than Monday at 2:33 p.m. EDT. Engineers need that time to troubleshoot an issue that resulted in today’s launch scrub. During today’s countdown, engineers detected a failure in one of two heater circuits associated with Auxiliary Power Unit (APU) 1. Heaters are required to keep the APUs’ hydrazine from freezing on orbit. Attempts to activate the heater were not successful and engineers now believe the problem might be associated with a Load Control Assembly, which is a switchbox, located in the aft end of Endeavour, or an electrical short in the wires leading into or out of the switchbox.