Follow the steps below to reset the toner light on your Brother HL-2040 Printer
Open the front cover and leave open while completing the following steps:
1.) Turn the printer off.
2.) Hold the 'go' button while turning the printer on. All panel lights should be on.
3.) Release the 'go' button once all lights are off.
4. )Press the 'go' button 2 times.
6.) Press the 'go' button 5 times.
7.) The toner light should be off.
8.) The paper light should be on.
9.) Close cover. The ready light should be the only light on.
Once all steps are successfully completed your "low toner" light should be fixed.
Taken from http://www.fixya.com/support/t143732-toner_led_remains_after_new_cartridge
Find additional tips like this one in our Brother Printer Tips section!