semiatic
Lo

Lo on a thermometer (below the measuring range)

The thermometer measured something colder than the lowest temperature it is built to report, so it shows Lo instead of a number. Every manual read treats it as a measurement to take again, not as a result: they tell you to check the probe or sensor and how the thermometer was held, then measure again.

Check your thermometer’s manual — makers differ.

What it is for

Measure again, following your manual’s steps. Braun (ThermoScan): “Make sure the probe tip and lens are clean and a new, clean lens filter is attached. Make sure the thermometer is properly inserted. Then, take a new temperature.” Microlife (NC 200): “Make sure the measuring sensor is clean. Then, take a measurement again.” Omron (ET720): “Use the unit within its measurable range. If a malfunction still exists, please contact OMRON customer service.” Beurer (FT 95): “Operate the thermometer only within the specified temperature ranges.”

These pages report what each maker’s manual says and quote it wherever health is involved. They do not diagnose anything, they are not medical advice, and they do not tell you whether a temperature is a fever or what to do about it; only your own thermometer’s manual and your doctor or nurse can do that for the person you are looking after.

What it looks like
The letters Lo, LO or just L where the number would be. Some makers show a word or an error screen instead.
Names makers use
Lo (Omron ET720); LO (Braun ThermoScan); L (Microlife IR 310, NC 200 and MT 410); Error (Withings Thermo); An error screen (Beurer FT 90 and FT 95, Braun No touch BNT 400)

Where makes differ

Where the floor sits depends on the maker and the mode. Omron’s ET720 manual shows Lo below 93.2°F (34.0°C) on the forehead and below -7.6°F (-22°C) in surface mode. Braun’s ThermoScan manual shows LO below 34 °C (93.2 °F), and Beurer’s FT 95 manual gives its error below 34 °C on the forehead and below 0 °C in object mode. Microlife’s floor is lower on some models: its manuals give L below 32 °C on the IR 310 ear thermometer and the MT 410 stick thermometer, but below 34 °C in body mode on the NC 200. Withings’ Thermo shows the word Error below 35°C (95°F). On a Microlife MT 410 stick thermometer an L with a flashing °C is normal before you start: Microlife’s manual says it appears when the air is below 32°C, and that the thermometer “is now ready for use”.

What it does not mean

Lo is not a low-battery warning: every thermometer read here has a separate battery symbol. It is not stored as a temperature either: Omron says its ET720 memory keeps the last results “excluding Hi/Lo results”.

Other symbols with the same shapes

HiHi on a thermometer (above the measuring range)The letters Hi, HI or just H where the number would be. Some…ErrErr or Er with a number (thermometer error code)Err, ERR or Er with a number where the reading should be. Braun…M2M, memory number or memory icon (a stored reading)An M, sometimes with a number after it, or a small memory icon…AHALRoom too hot or too cold (AH, AL or ambient message)The letters AH or AL on Microlife thermometers; an error screen…POSPOS, ERP or Er 2 (thermometer not held in place)Short letters such as POS or ERP instead of a reading, often with…

Sources

No published standard we could check defines the symbols on a digital thermometer’s display, so this meaning is taken from 5 makers’ own manuals and support pages, each fetched and read on the date shown:

Written in our own words; quotations are the makers’ wording.