Why does my braun irt 4520 say lo on the screen,it has new batteries and I cleaned the plastic covering but it won't do nothing but flash the lil sign for the covering

Inspect the battery contacts for a little oxydation sometimes if its on the battery-spring contact it can happens OKS 1110 or Molykote 111 are very good protectors on battery contacts everywhere it will give long lasting protection also on the both contacts of the battery self.
In rooms with moisture it gives long lasting protection for energy-efficient appliances where batteries longer than average to be placed.

Have you tried replacing the disposable-lens filter ?
The manual is available at: http://brauntherms.com/downloads/IRT4020_4520_MNLA_U.0035_OM_25JAN11.pdf

this is manual for 4520.... Braun Ear Thermometer Owner Manual ManualsOnline com

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.?

this is manual for 4020 Braun Thermometer Owner Manual ManualsOnline com

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.

Problem:
Temperature taken is not within typical human temperature range (93.2 °F - 108 °F / 34 °C - 42.2 °C).

Solution:
Make sure new, clean lens filter is properly inserted. Then, take a new temperature.

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.