Output 2 is a supervised siren output. If there is nothing connected to it, or if the device connected has a broken or cut wire, an Output 2 Trouble occurs. If Output 2 is not being used, a 2.2k ohm resistor should be installed across the Output 2 terminals
Try installing the 2.2KΩ Resistor across the + and – of the siren. When Output 2 is set to Voltage (Globals 26) it is rated for 12 VDC at 1 AMP max current and you should make sure your self-contained siren does not draw more than 1 Amp.
Yes many installers have to come to grips with the nature of an output being supervised with a resistor across it. That 2.2KΩ does the trick even if you are pushing something like a Doberman horn/strobe. I have gone a step further though Brad and I have found another issue. I have a 100 foot fire wire run (or less; loop ohms 2.3Ω) with the Global 26 set to voltage output (which was running the Doberman fine) switched out to a self-contained System Sensor P2R set for 15 Candela and medium temporal 44mA and it would barely chirp. I was puzzled. Started trouble-shooting by removing the Sensor and measuring the voltage across the the pair when we fired off the Fire Alarm. Found the voltage bouncing from 13.8 V down to 4.3V then back up again. So I applied 12 V straight from the ELK-412 that I have with the system and the P2R sounds great. So I believe even though the Output 2 is suppose to be a straight Voltage Out it is actually doing a Fire-Yelp type signal voltage which sort of makes sense but does not satisfy the need for Self-Contained fire appliances. Yep you know me. Always trying to make this thing do something it was not designed for :). Figured this discussion might help someone else starting to deal with heavier fire install situations using the new Elk-6052 or 6053 Smoke /Heats. Which work great!!