The command and service modules were mated in the KSC altitude chamber in September, and combined system testing was performed. Altitude testing was performed first uncrewed, then with both the prime and backup crews, from October 10 through December 30. During this testing, the environmental control unit in the command module was found to have a design flaw, and was sent back to the manufacturer for design changes and rework. The returned ECU then leaked water/glycol coolant, and had to be returned a second time. Also during this time, a propellant tank in service module 017 had ruptured during testing at NAA, prompting the separation of the modules and removal from the chamber so the service module could be tested for signs of the tank problem. These tests were negative.