I have a Braun IRT 4020 Thermoscan ear Thermometer. As there was no display when I pressed the start button , I changed the battery after which only the OFF is seen on the disply after which I pressed the start button once more and there was the degreeC sign, SET, degreeF display on the unit, after which it returns to the OFF and stays that way. What should I do?

Question edited for clarity, and spelling 'brain' to Braun, and 'thermoter' to thermometer, 'comes in' to comes on..
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There are some ideas here, but it sounds faulty and needs warranty return or repair.

https://www.google.com/search?q=Braun+thermoscan+hm3+not+working

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1. Memory mode.
2. IRT 3020. The last stored temperature is displayed along. with the MEM symbol. ...
3. IRT 3520. This model allows you to store up to 8 temper- atures. ...
4. Memory clear. Press the /mem button for 5 seconds to clear. the temperatures stored in memory. ...
5. LCD light (model IRT 3520 only) For easy nighttime reading.

https://www.manualsdir.com/manuals/50884/braun-thermoscan-irt-3020-thermoscan-irt-3520.html?page=9

is the drawer that its stored in, in the bathroom? the humidity or moisture could affect it. I have had mine for 8 yrs and works perfectly. Are you placing the probe covers on correctly and waiting for it to click or a pop before inserting into ur ear.?

You did not provide a model number however the Instruction Manuals for the various Braun Thermometers can be found here:

http://braun.kaz.com/kaz/service/prodResult/

Not sure if this procedure is common to all but this may work otherwise find the manual specific to your model number.

Changing the temperature scale

This thermometer is shipped with the Celsius temperature scale activated. If you wish to switch to Fahrenheit ( °F), proceed as follows:

Turn on the thermometer. ( If it is already turned on, make sure it is not in memory mode.)

Press the /mem button and keep it pressed. Then press and release the activation button to switch over to the «change temperature scale» mode.
By pressing the activation button again, the Fahrenheit scale is activated, «°F» is displayed on the LCD. Each further pressing of the activation button toggles the scale between °C and °F.
Release the /mem button to return to the ready symbol . The temperature stored in memory will automatically be converted to the selected scale when displayed on the LCD.

If you whish to switch from Celsius to Fahrenheit, proceed as follows:

(1) Make sure the thermometer is turned off.

(2) Press and hold down the Start button (IRT 4020)
or the «I/O» button (IRT 4520). After about
3 seconds the display will show this sequence:
«°F» / «SET» / «°C» / «SET» ...

(3) Release the Start button / «I/O» button when the
desired temperature scale is shown. There will be
a short beep to confirm the new setting, then the
thermometer is turned off automatically.

Turn off the thermometer
Press and Hold the L/O Button
Continue to hold the start button when the F or C display appears. Watch for these displays to cycle over the display window.
#
Release the start button when the F display appears on the screen.
#
5

Listen for a beep and watch for the Braun ThermoScan ear thermometer to shut itself off, indicating that you changed the temperature scale to Fahrenheit successfully.
#
6

Follow the same procedure but release the start button when C shows on the display to switch from Fahrenheit to Celsius.

Changing the temperature scale Your Braun ThermoScan is shipped with the Fahrenheit (°F) temperature scale activated. If you whish to switch to Celsius (°C) and/or back from Celsius to Fahrenheit, proceed as follows: (1) Make sure the thermometer is turned off. (2) Press and hold down the Start button (IRT 4020) or the «I/O» button (IRT 4520). After about 3 seconds the display will show this sequence: «°F» / «SET» / «°C» / «SET» ... (3) Release the Start button / «I/O» button when the desired temperature scale is shown. There will be a short beep to confirm the new setting, then the thermometer is turned off automatically.

... call Braun ThermoScan customer service at 1-800-327-7226

Changing the temperature scale Your Braun ThermoScan is shipped with the Fahrenheit (°F) temperature scale activated. If you whish to switch to Celsius (°C) and/or back from Celsius to Fahrenheit, proceed as follows: (1) Make sure the thermometer is turned off. (2) Press and hold down the Start button (IRT 4020) or the «I/O» button (IRT 4520). After about 3 seconds the display will show this sequence: «°F» / «SET» / «°C» / «SET» ... (3) Release the Start button / «I/O» button when the desired temperature scale is shown. There will be a short beep to confirm the new setting, then the thermometer is turned off automatically.