Brother HL-5140 - New Toner, Print is very light/Poor quality. - Haven't used printer for several months. The only problem then was it needed a new toner cartridge. I had another printer, so I didn't rush out to buy a replacement. I have since started using it again and replaced toner cartridge with a Brother TN-570, the print was very light and seemed to fade out even more towards the left margin. Thinking the cartrige may have been bad, returned it for another, TN-540, and same thing is happening. There ar no lights flasing on the control panel, only light on is the green status light. Been messing with it for about a week! What's Up? I'm at a loss. PLEASE help!

With the front cover open, press and hold down Go button until all 4 LEDs are lit. Once all 4 LEDs are lit, release the go button, then close the front cover.

This will reset the drum counter.

DRUM UNIT "spent"
It can be reset if the printing quality is good
To reset the drum follow the instructions below:
Press the cover release button and then open the front cover of the printer.

Press and hold down Go button until all 4 LEDs are lit. Once all 4 LEDs are lit, release the Go button.

Close the front cover.
Make sure that the Drum LED is now off.
Caution: Reset the drum counter only when you replaced the drum unit with a new one.

Sadly this happens all the time - we have about 12 of these printers and 2 have the same problem at this moment and the only fix we have found previously is to replace the drum unit with a brand new one which is hardly economical - indeed the machines are all NHS funded and often the solution has been to throw the printers with this problem away and we have a few defunct ones piled in a corner because of this - if anyone knows why and how this happens ( the tech dept locally has no ideas but just wupply an new printer as they are cheap) I would be very interested also - changing the drum and cartride to a partly used one from a different machine does nothing at all either so it has to be something to do with the chip in the new drum resetting the machine but how and why is beyond us. In every case the printer prints perfectly before the fault and after the change to a new drum so I can only assume it is a way of generating more drum sales??

Try this. 
http://welcome.solutions.brother.com/BSC/public/files/dlf/doc000071/HL5100_UG_ENG_3.pdf

I have reused the drum for three life cycles and the print quality is still good. Just reset the counter until the print quality suffers because the cost of the drum exceeds the price of a new printer. According to Brother to reset the counter

1. Turn the printer off.
2. Press and hold the Go button on the front of the printer, at the same time turn the printer power on.
3. When the Toner light is on by itself, release the Go button.
4. Push and hold the Go button again, release the Go button when the Paper light is on by itself.

Sounds like a fusing error ... If your able to wipe the toner off the page its a fuser unit problem (replace fuser unit) If toner is sticking to paper but the print quality is poor , replace the drum unit  (PCU)  good luck

The red lite indicates one of the following needs to be looked at:

Cover open (Front cover) Close the front cover.

Cover open (Jam clear cover) Close the jam clear cover of the fuser unit.

Memory full - Press the go button.

Print overrun - Press the go button.

Clean drum unit - Clean the corona wire.

Are you using brother gen toners?

Are you using recycled toners, as these have a habit of spilling toner out? Take the toner unit out of the drum and look at the green roller in the drum to see if that has the toner marks on it.
Don't hold this unit in the light too long as it can damage it.

no other lights if you hold start/go button then replace toner with Gen New one