Hello. The printer model number is Officejet 6500 Wireless. The operating system on my laptop connected to the printer is Mac OS High Sierra. The problem is the printer will not print black only colour. The colours are working perfectly but despite following the troubleshooting instructions on the HP website the printer will still not print black! When I used Cartridge Alignment on the printer it said the alignment had failed! Please help urgently as I use my printer every day for work so need to get this resolved as quickly as possible! Thanks in advance for your help and advice.