Beach:Fisher CZ-20, Beach Hunter ID 9.5" Whites DFX, Minelab Safari and Excalibur 2.
Park and Turf: Teknetics Gamma 6000,Teknetics Delta 4000,Nokta Fors Core
Loaners:ACE-250 9x12 and 7x9.
Set sensitivity to maximum, set elimination to 0 and threshold to 2-3 go outside and airtest the unit. The elimination feature is worthless and you lose lots of depth. You should hear a break in the threshold at roughly 12 to 13" with the 8" coil when passing a quarter in front of the coil in an even somewhat rapid steady manner. The key is listening to the very low whispers that break the threshold at the mentioned distance. If you hear the break your unit has proper gain. At 9" it should be pretty loud and a noticeable break/sound. I had one and was a great unit, very well built and deep but they will not "discriminate" iron without losing gold.
Hit it several times with the Garrett A53 Hammer. If it still powers up, continue to vigorously hit it with the A53 until it doesn't. Step Two: Purchase a Tesoro Sand Shark immediately, and start enjoying the beach like a REAL pirate!
Have you put in fresh batteries? Have you checked the coil connections? Is this an air test or real world? Make sure you aren't using any discrimination and that there isn't some electrical device interfering with the detector. If that fails, give Garrett a call -- their factory service is probably the best in the industry.