Omron thermometer screen: what Er1, Er2, Er3, Hi, Lo and the other messages mean
Omron’s instruction manual for the No-Touch Forehead Thermometer ET720 (model MC-720-Z) names every state below. Each one has Omron’s own words in quotation marks, what Omron says to do, and a link to our page on the same symbol across other makes.
Check your thermometer’s manual — makers differ.
All 13 Omron screen states at a glance
| On the screen | What Omron’s manual says it means |
|---|---|
Er1 | Measured before the thermometer was ready |
Er2 | The room temperature around the thermometer changed quickly |
Er3 | The room is too cold or too hot to measure in |
Errors 5 – 9 | A fault in the thermometer itself |
Hi | Above the range it reports |
Lo | Below the range it reports |
Battery icon | The battery is low |
Every segment stays lit | The battery is used up |
Number left on the screen | Room temperature, not a reading |
Surface icon | Surface mode: an object’s temperature, not a body temperature |
Mute icon | The buzzer is off |
Memory icon and a number | A stored reading |
OFF | Switching off, back to the room temperature |
Which models this covers
The manual read here is Omron Healthcare’s US edition for the ET720 (document 5681249-5D). Its error table draws each code as a picture of the screen, so the codes named here (Er1, Er2, Er3, Er, Hi, Lo) are read from those pictures and the meanings from the words beside them. Omron sells other thermometers, such as stick and ear models, with their own manuals; their codes may differ.
Each state, in Omron’s words
Er1 Measured before the thermometer was ready
“Measurement before unit is ready.”
What Omron says to do: “Wait until all the icons stop flashing.”
Er1 is about timing, not about you or the thermometer: after you press [ON/MEM], the manual says all indicators show for 1 second and the ready screen follows with two beeps. Start the reading after that.
Every make: Flashing °C, °F or mode symbol. Source: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.
Er2 The room temperature around the thermometer changed quickly
“The unit is showing a rapid ambient temperature change.”
What Omron says to do: “Leave the unit in a location where the temperature is between 50 and 104 °F (10 and 40 °C) for at least 30 minutes.”
This is the code to expect after carrying the thermometer from a cold car or a warm pocket into the room. Omron’s questions and answers add that a unit stored in a cold room may read rather high if used soon after.
Every make: Room too hot or too cold. Source: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.
Er3 The room is too cold or too hot to measure in
“The ambient temperature is not within the range between 50 and 104 °F (10 and 40 °C).”
What Omron says to do: “Leave the unit in a location where the temperature is between 50 and 104 °F (10 and 40 °C) for at least 30 minutes.”
Er2 and Er3 share one fix in Omron’s table. Er3 is about where you are measuring, Er2 about how fast the temperature changed.
Every make: Room too hot or too cold. Source: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.
Errors 5 – 9 A fault in the thermometer itself
“Errors 5 - 9 indicate a malfunction of the unit.”
What Omron says to do: “Remove the battery, wait for 1 minute, and reinsert the battery again. If the message still appears, please contact OMRON customer service.”
Er1 to Er3 are fixed by waiting or moving; Errors 5 to 9 are the ones Omron says to take to its customer service if a battery restart does not clear them. Omron’s table has no Er4.
Every make: Err or Er with a number. Source: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.
Hi Above the range it reports
“If the result is higher than 108.0 °F (42.2 °C), Hi appears on the display.”
“Surface measurement mode: The temperature reading is higher than 176 °F (+80 °C).”
What Omron says to do: “Use the unit within its measurable range. If a malfunction still exists, please contact OMRON customer service.”
The ceiling depends on the mode: 108.0 °F (42.2 °C) on the forehead, 176 °F (80 °C) when measuring an object’s surface.
Every make: Hi on a thermometer. Source: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.
Lo Below the range it reports
“If the result is lower than 93.2 °F (34.0 °C), Lo appears on the display.”
“Surface measurement mode: The temperature reading is lower than -7.6 °F (-22 °C).”
What Omron says to do: “Use the unit within its measurable range. If a malfunction still exists, please contact OMRON customer service.”
Lo is not stored: Omron says the memory keeps the last 25 results “(excluding Hi/Lo results)”. Its questions and answers list wet skin among the reasons a forehead reading comes out rather low.
Every make: Lo on a thermometer. Source: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.
Battery icon The battery is low
“Battery is low.”
What Omron says to do: “Replace the battery.”
The ET720 runs on one CR2032 lithium button cell; Omron gives approx. 2,500 measurements or more from a new one.
Every make: Low battery symbol. Source: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.
Every segment stays lit The battery is used up
“Remains on the display.”
“The battery is depleted.”
What Omron says to do: “Change to a new battery.”
Omron’s table draws this state as the full screen with every digit and icon showing. A moment of every segment at switch-on is normal; a screen that stays that way is the battery.
Every make: Low battery symbol. Source: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.
Number left on the screen Room temperature, not a reading
“If the thermometer is not used, after 1 minute the device will automatically switch to room temperature measurement mode, and the room temperature will appear on the display.”
“The room temperature remains on the display even after the unit turns off.”
The number left on the screen is the room. Check for the forehead icon before you read a number as a temperature.
Every make: Body, forehead, object or room mode symbol. Source: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.
Surface icon Surface mode: an object’s temperature, not a body temperature
“Surface measurement mode shows the actual and unadjusted surface temperature of an object, which is different from the body temperature.”
“Surface measurement mode is not intended for body temperature measurement or medical use.”
Surface mode is entered by holding [ON/MEM] and pressing [START]. Omron adds that the display does not light up and the buzzer is off in this mode, and that a minute without use takes the thermometer back to room temperature mode.
Every make: Body, forehead, object or room mode symbol. Source: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.
Mute icon The buzzer is off
“The icon remains lit and the buzzer is deactivated.”
What Omron says to do: “To activate the buzzer again, start from step 1.”
Turning the buzzer off is done by holding [ON/MEM] for 3 seconds until the icon flashes. Omron notes the buzzer only works in forehead mode.
Source: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.
Memory icon and a number A stored reading
“This unit automatically stores the last 25 measurement results (excluding Hi/Lo results).”
“The memory number appears on the display.”
Pressing [ON/MEM] again steps back through older results. Switching between °F and °C deletes every stored reading.
Every make: M, memory number or memory icon. Source: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.
OFF Switching off, back to the room temperature
“Press and hold the [ON/MEM] button until OFF appears on the display.”
After OFF the display goes blank briefly, then shows the room temperature again; that is normal.
Source: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.
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.
Other thermometer makers
Sources
Every state above is quoted from this document on Omron’s own website, fetched and read on the date shown. Words in quotation marks are Omron’s; everything else is ours.
- Omron: Omron No-Touch Forehead Thermometer ET720 (MC-720-Z) — instruction manual (US, 5681249-5D, PDF) Checked 2026-10-13.