Yesterday I attended a HP M570dn printer repair in EC2 with poor print quality on the black colour. This problem can be caused by a number of issues. The first part of the printer that needs to be checked is the toner as this can be faulty in 2 ways. One way in which the toner can be faulty is that it's not releasing toner quick enough.The second fault can be that its leaking toner from the cartridge, meaning the cartridge will need to be replaced and the inside of the machine will need to be cleaned. On this specific job the problem was leaking a cartridge, if this wasn't the case the next step would be to change the black drum and then transfer belt.