It forced call takers and dispatchers on to use paper and pencil to take down information.
The system was stalled for a total of 45 minutes Monday, but fire officials said it appeared to be back on track by afternoon.
The giant 911 system is in the process of being upgraded at a total cost of $2 billion, including a new back-up center.
The piece that failed Monday was EMS dispatch, a system from the 1980s that's scheduled to be upgraded in 2015.
There have been glitches in other, newer parts of the system that have prompted criticism.
Fire department workers were running diagnostic tests to determine why the system was having problems.