I replaced the battery in my safety 1st ear thermometer #49551. It still reads lo battery and will not operate.

It's probable that it needs a new lens filter or the filter needs cleaning. I have no idea how to remove the filter but it probably clips or screws off the bit that goes in the ear.
Quite frankly the device is at best unreliable, it measures infra-red in the ear, great in theory but not very reliable in practice. I would use a different thermometer.

Have you tried replacing the disposable-lens filter ?
The manual is available at: http://brauntherms.com/downloads/IRT4020_4520_MNLA_U.0035_OM_25JAN11.pdf

I just had that LO reading. Pulled back on the tip of the thermometer. Seems I had it inserted into my turkey too far. Probably up against a bone or too close to the center. Good luck! Happy Thanksgiving 2016!

Uhm, you're in the Humidifiers section, you may want to move this question to another section...

there should be a setting on it that changes the temp from Celsius or "c" to ferinheight and lo means that it currently isn't reading a temperature in the normal range. try running it under hot water and see if it says error or high.

Go to http://www.safety1st.com/usa/eng/Products/Child-Care/Thermometers/?start=0 and find your thermometer. Click on the picture, towards the bottom there is a section you can click on called instructions. Then it will bring up a new menu it will ask you to enter your product type, model, and manufacture date and you can download the instructions from there. I hope this helps.

According to the 10428 manual from Safety 1st:
"When the battery is exhausted. Dispose of the thermometer as the battery is non-replaceable. Be aware of any local regulations concerning disposal of this product. The battery cover
is sonic-welded to the main housing of the thermometer, the battery is well sealed inside the thermometer to reduce the risk
of being swallowed by children."

According to the 10428 manual from Safety 1st:
"When the battery is exhausted. Dispose of the thermometer as the battery is non-replaceable. Be aware of any local regulations concerning disposal of this product. The battery cover
is sonic-welded to the main housing of the thermometer, the battery is well sealed inside the thermometer to reduce the risk
of being swallowed by children."

when its offhold the button down until you see the f pop up then release.