Brother HL-2140 does not print

Even if the toner is new or full, the Brother HL-2140 stops printing. This is one common problem to this model. Here is what you need to do:
1. Turn off your printer
2. Take out the toner.
3. On both sides of the toner, you will see a small hole - this is the sensor.
4. Cover the sensor with masking tape and color the tape with black marker. you can also use electrical tape to cover them.

Here is a detailed video on how to do it:

Good luck!