Commander Mark Kelly and his crew were on their way to the launch pad, when NASA halted the countdown. The astronauts' van did a U-turn, and returned the astronauts to crew quarters.
NASA spokesman George Dillard says the next try will be Sunday at the earliest.
NASA reported earlier that two heaters on an auxiliary power unit were not working. Engineers could not understand the problem, and the launch was halted.
Commander Kelly is married to wounded congresswoman Gabrielle Giffords. She is already in Cape Canaveral. President Barack Obama planned to watch the launch but had not yet arrived.
Engineers were still working to figure out what caused the problem.
Cloudy weather and strong winds had earlier increased the likelihood of the launch of the space shuttle Endeavour being delayed from 20 to 30 per cent.
The penultimate shuttle liftoff is drawing hundreds of thousands of onlookers to the area around Cape Canaveral.
The 30-year space shuttle program will end after the final launch by Atlantis, planned for late June.
