You could just give them a call and get the right answer the first time.
It's possible there is something wrong with the coil or the wire or the batteries are low. Either way they will help you find out the problem. They have great customer service.
http://www.tesoro.com/service/contact/contact.html
Whether you have technical questions, repair concerns, or just need some advice on choosing the right detector, we are here to serve you. Thank you. Tesoro Electronics
715 White Spar Road
Prescott, AZ 86303
Tel: 928-771-2646