STS-31 was launched on 24 April 1990 at 8:33:51 am EDT. A launch attempt on 10 April was scrubbed at T-4 minutes for a faulty valve in auxiliary power unit (APU) number one. The APU was eventually replaced and the Hubble Space Telescope's batteries were recharged. On launch day, the countdown was briefly halted at T-31 seconds when "Discovery"'s computers failed to shut down a fuel valve line on ground support equipment. Engineers ordered the valve closed and the countdown continued.