The water heater has multiple control features that ensure safe and proper operation.  If the unit detects an issue, it will shut down and display an error code on the front display.  To reset the water heater, press the RESET button on the front control panel.  On newer units the reset button is labled as a return arrow.  The unit will attempt to restart after resetting, and may resume operating.  If conditions still exist that prevent operation the error code will reappear.  Any time an error code appears on the unit, a service contractor should verify the proper set up and operation of the water heater.