Lo

Various Digital Thermometer

Severity: Minor

What it means

Digital thermometer 'Lo' on the display means the device cannot measure the temperature because what it's pointed at (or in contact with) is below its measurement range.
Philips Avent documents it as: 'Lo indications on the display mean that the temperature cannot be measured because the temperature is outside the temperature range.'
For body thermometers, Lo usually means the device hasn't gotten a stable contact yet, or the body part is genuinely cold (e.g., hand-held probe before insertion).
For ambient or food thermometers, Lo means the target is colder than the device can read.

Affected Models

Body thermometers (oral, ear, forehead) — Philips Avent, Braun ThermoScan, Vicks, Welch Allyn
Smart thermometers — Kinsa Smart Ear, Withings Thermo
Cooking and meat thermometers (instant-read and probe types)
Pharmacy / consumer thermometers (Andatech MedSense, Apollo brand, generic chinese OEMs)
Lo behaviour is universal — every digital thermometer with a measurement floor shows Lo (or 'L', 'LO') when the target is below that floor

Common Causes

Probe not yet in contact with the body or in correct position (most common on first reading)
Body part is cold (hand-held probe pre-insertion, just-undressed armpit)
Food or surface is colder than the thermometer's lower limit (most consumer probes are -10°C / 14°F to ~50°C / 122°F)
Battery low — some thermometers show Lo when the battery can't sustain a reading
Probe tip damaged or contaminated — readings drift below the actual temperature

How to Fix It

Position the thermometer correctly.

Oral: under the tongue, mouth closed, wait the full beep.
Ear (tympanic): pull ear up and back, insert probe firmly, hold still for the full measurement.
Forehead (non-contact infrared): hold at the distance the manual specifies (usually 3-5 cm), aim at the forehead's centre.
Armpit: dry the skin first, probe deep into the armpit, hold arm firmly against the body.
Wrong position is the #1 reason Lo appears.
Wait the full measurement time.

Different thermometer types need different settling times: oral 30-60 seconds, ear 1-3 seconds, forehead immediate, armpit 3-5 minutes.
If you remove the thermometer too early, you get Lo because it hasn't had time to detect peak temperature.
Wait for the beep or the steady-temperature signal.
Warm the probe area on the body.

If the probe tip is colder than room temperature (came out of a cold cabinet or was wiped with alcohol), it needs to equalise.
Hold the probe in a closed hand for 30 seconds before taking the reading.
This is most relevant for surface thermometers and oral probes — body thermometers are designed to read fast, but a cold probe still falsely shows Lo briefly.
Replace the battery.

Many digital thermometers show Lo as a low-battery warning, especially when the battery is too weak to drive the heating sensor circuit.
Open the battery compartment (usually a small slide-off cover at the back of the handle) and replace the button-cell battery — most use CR2032 or LR41.
If Lo persists with a fresh battery, the issue is the thermometer itself, not the battery.
Clean the probe tip.

For ear and forehead infrared thermometers, the lens at the probe tip needs to be clean.
Wipe gently with a soft cloth lightly moistened with alcohol (no scrubbing, no harsh detergent).
A dirty lens reads cooler than reality and gives Lo on healthy targets.
Let dry fully before next use.
Replace the thermometer if Lo persists.

If a fresh battery and clean probe still give Lo on a healthy target, the sensor or the internal circuit has failed.
Most digital thermometers are inexpensive — replacement is usually the right call rather than repair.
Keep the old one for parts (the case and the button battery) but use a new device for actual readings.

When to Call a Professional

Lo is almost never a fault on a body thermometer — it just means the device is waiting for a proper reading.
If Lo persists after correct positioning and a full warm-up period, the thermometer or its battery may be at the end of life.
For body temperature measurement, if you genuinely need a reading and Lo won't clear, switch to a backup thermometer rather than trusting an erroneous result.

Frequently Asked Questions

My forehead thermometer always shows Lo on my baby — is the baby's temperature really that low?

Almost certainly not — babies' core body temperature is the same range as adults (around 36.5-37.5°C / 97.7-99.5°F).
What's far more likely: the forehead thermometer is being held too far from the skin, or the skin is cold from being outside, or the lens is dirty.
Move the device closer (3-5 cm from skin), hold for 3 seconds, then take the reading.
If Lo persists, try an ear or armpit thermometer as a sanity check — a true 'low' body temperature on a healthy baby should be cross-confirmed before assuming the reading is correct.