Update - The County of San Luis Obispo says during the annual test, 130 sirens operated successfully.
However, one siren, near Highway 101 and El Campo Road just south of Arroyo Grande did not sound.
PG&E and the County of San Luis Obispo say they are investigating the malfunction.
In the meantime, backup alert and notification procedures have been put in place for that area should the sirens be needed in an emergency.
San Luis Obispo County’s Early Warning System sirens will be tested this Saturday, Aug. 21.
The annual test will take place at noon and again at 12:30 p.m.
The sirens will sound for three minutes each time.
The siren system is tested throughout the year, but only once a year do all sirens sound at the same time.
The sirens are located from Cayucos to the Nipomo Mesa and can sound for any local emergency that requires people to take action, such as a tsunami or nuclear incident.
If you hear the sirens at a time when a test is not planned, turn on your local radio or television station for emergency information and instructions.
For more information on emergency preparedness, click here.