In general terms, if a waste toner message persists even after waste toner has been emptied, it is one of two things:
1. the bane of laser printers of all kinds in all times and places: loose toner. Possibly over the sensor that tells when the waste toner bottle is full - in which case, it still thinks it's full. Polish everything in there that could be a sensor (with the power off and unplugged of course, for safety), and if the waste bottle's smeary with toner still, rinse it out I guess, allow it to dry, and put it in again after it's dry.
2. Maybe it needs a reset or acknowlegement button press or sequence. I will now look that up briefly... comment or respond if you want meantime, I will be back soon.
Brother's website suggests that the last resort if the waste toner error message won't clear, is to get a 'new waste toner' - part # XXXXX
So it seems from this webpage that it's not enough to have emptied it, you should also replace it:http://www.brother-usa.com/FAQs/Solution.aspx?FAQID=200000015345&Sympt=Error+on+printer+LCD%3a+Waste+Toner+Full&Model=1052&ProductGroup=1&Keyword=waste&ProductID=HL4040CNthe complete procedure, step-by-step, of replacing the waste toner 'box' is detailed on that page.
From where do i get one, do you know?
Brother's website or your local super store. The part # XXXXX XXXXX Brother part. I will refer you a web page at Brother's site in a moment... but first this:Brother advises if the error message persists, you might want to:"Check and clean the two metal contacts on the back of the waste toner box."
As for the part, to buy it: http://www.brother-usa.com/accessories/Accessories.aspx?Model=HL4040CN&ACASCID=35it's the second of the two choices on that page. Price: $26.49
If this helps with the problem and answers your question, then please click once on Accept for my Answer. (Only then will I be paid, by deduction of my share from the funds already deposited by you with JustAnswer. Thanks.) - Favorable feedback is appreciated.