I have 2 of these meters, the 840 and 845 purchased last year and no problems.
I bought 2 others including this one and 530 (the equivalent but wired) last month and actually, nothing to do:
The battery is bad contact inside (remedied with a piece of duct tape under the hood) then there 'now is play the counter once laid, again, a thin layer of sticky for extra thickness, and after more no problem.
However, the one thing that has never changed is a certain slowness in the interface, but it is known by reading right to left.
Otherwise, a super meter, very detailed, with 3 simultaneous displays (so tachometer, resistant to rain and falls, checked a few times.