Then After putting in some used batteries, it wasn't right but lit up....however, we found a battery tester, and put in two new good batteries, and it's back to working.

Put in new batteries and still doesn't work.

Have no idea how this works because the batteries are unusable

I even tried fresh batteries, but it won't work

Out of the box with fresh batteries, the device measured pulse and oxygen about 20% lower than the device used in my doctor's office.

Very easy to use came with batteries ready to work

I placed the batteries in the machine and almost burned and caught on

Has a battery signal as well.

I tried replacing the batteries and still does not work.

Tried to change batteries and still doesn't worth.

Works right out of the box as long as you put the batteries in the correct way, just read the instructions on battery placement and do not put them in in the conventual way.

New batteries do not help the issue.

Also, the batteries they included were essentially dead.

This is the most stupid battery holder I've seen

I can not vouch for accuracy over time, but I imagine that there is not a lot that could cause that to degrade other than maybe a failing battery or dirty sensor

I even tried fresh batteries, but it won't work

It was broken as if someone stepped on it.

Works as it is supposed to