Message "Back cover open. Close back cover" on Brother Dcp 7020 printer

Hi,
This error is due to tray problem check if the back tray latch is not broken .also check the Fuser unit at the back if its not been fixed or loose contact.
Regards.

I was about to surrender, but tried again in a calmer mode and started checking everything in the paper path thru the printer.There's a small coiled spring that can come loose under the printer. This spring is connected to the plastic lever/sensor that tellls the printer there's paper in the drum/toner chamber.You should see it with the help of a lamp or flashlight, since almost everything under the printer is black plastic. You have to prop the printer on it's back to see it. there'a metal plate exactly below the paper feed rollers, you have take that plate out by removing the six screws that hold it in place. This is to check that the lever/sensor I pointed before is actually inside the hole that holds it in place. The spring, if its loose should be hanging by one of it ends using very small tweezers or needle nose pliers, you should be able to hook it back to the lever/sensor, it has a hooked end where the spring should hook to. The printer is working like a charm again!!! (IF THIS SPRING IS MISSING, IT SHOULD BE REPLACED WITH A SIMILAR ONE, IT IS ABOUT THE SIZE OF A BALLPOINT PEN SPRING, BUT WITH LOW TENSION, IT HAS THE TENSION OF A LEAF SPRING LIKE THE ONE IT THE BACK COVER OF THE PRINTER)