600amp clamp problem

Hi i tried to use my 600amp clamp(TA019) yesterday, when i switch it on it reads off the scale on 600amp setting, or reads 5.6volts on voltage scale without being connected to car
My question is can these be tested/fixed or is it not worth the bother should i just order a new one.
I did open it last night but didnt see anything that looked damaged so put back together again.
Ps the first thing i tried was a new battery and still the same.
Regards Damian

Ordered a new one today
Damian