Digital Thermometer 'Hi' Reading — What It Means & Fix

What it means

Digital thermometer 'Hi' on the display means the target temperature is above the measurement range of the device.
Philips Avent documents it as: 'Hi indications on the display mean that the temperature cannot be measured because the temperature is outside the temperature range.'
For body thermometers, a genuine Hi reading means temperature exceeded 42-43°C (107-109°F) — that's a medical emergency.
More often, Hi is caused by incorrect positioning, just-eaten hot food affecting an oral reading, or a thermometer pointed at a hot lamp or heater by mistake.

Affected Models

Body thermometers (oral, ear, forehead) — Philips Avent, Braun, Vicks, Welch Allyn, Withings
Smart thermometers — Kinsa Smart Ear (which documents Hi/Lo recovery in its support)
Surface and cooking thermometers (instant-read meat probes, candy thermometers)
Pharmacy thermometers — Andatech MedSense documents Hi/Lo behaviour
Hi behaviour is universal — every digital thermometer with a measurement ceiling shows Hi when the target exceeds that ceiling

Common Causes

Genuine high fever — body temperature actually above the device's range (medical emergency)
Just-eaten hot food or drink affecting an oral reading
Forehead just exposed to direct sun, heat lamp, or hairdryer
Forehead thermometer pointed at a heat source by mistake (lamp, radiator, hot wall)
Probe contamination — residual hot substance on the tip

How to Fix It

Don't dismiss a Hi reading on a person.

Hi on body thermometers means temperature above ~42°C / 107°F.
That's potentially life-threatening — seek emergency medical care.
But before you assume the worst, take a second reading 5-10 minutes later with the same thermometer in a different position (or a different thermometer if you have one).
If the second reading is normal, the first was likely false.
If the second confirms high fever, call emergency services.
Wait 30 minutes after hot food or drink for oral readings.

Hot coffee, hot soup, or even very warm food can raise oral temperature by 1-2°C briefly.
If the person just ate or drank something hot, wait 30 minutes and retake the reading.
This is a known false-Hi source on oral thermometers.
Move away from heat sources.

For forehead infrared thermometers, point them at a cool wall instead of the person's forehead.
The thermometer should display approximately room temperature.
If the reading on the wall is also too hot, the thermometer was pointed at a heater, lamp, or sunlit surface — find a cooler area and retest.
Clean the probe tip.

Wipe the probe tip with a soft cloth lightly moistened with alcohol — no harsh chemicals.
For oral thermometers especially, food residue, drink residue, or sterilising agents left on the probe can cause Hi readings.
Let the probe dry to room temperature (about 5 minutes) before the next reading.
Check the probe hasn't been damaged.

If the probe tip is cracked, melted, or burned, the sensor inside can read incorrectly.
Visual damage to the tip means the thermometer needs replacement — there's no safe way to fix sensor damage on a body thermometer.
Replace the battery.

Low battery can cause sensor circuit instability that reads spuriously high.
Replace with the correct button cell (CR2032, LR41, depending on model).
Take a known-good reading (e.g., your own forehead while feeling fine) — should read normal body temperature.
If it still reads Hi, the thermometer itself has failed.

When to Call a Professional

Hi on a body thermometer is potentially serious and shouldn't be dismissed.
True body temperature above 41°C / 106°F is a medical emergency requiring immediate care.
Before assuming a false reading, take a confirmation reading 5-10 minutes later with a verified-clean probe in a verified-cool environment.
If the second reading also shows Hi or a temperature above 40°C, seek medical attention.

Frequently Asked Questions

How do I tell if Hi is a real fever or a false reading?

Take a second reading 5-10 minutes later with the person at room temperature and the probe verified clean.
If the second reading is normal (36-37.5°C / 97-99.5°F), the first reading was false — likely caused by hot food, drink, recent exercise, or sun exposure.
If the second reading also shows Hi or any number above 40°C / 104°F, treat it as a real fever and act accordingly: for an adult with a temperature above 41°C / 106°F, seek emergency care; for a baby under 3 months with any fever, seek immediate medical advice; for older children, follow your pediatrician's guidance.
Don't dismiss a genuine Hi reading just because you suspect the thermometer is wrong — verify, don't assume.