They will walk you through some tech support to make sure it really needs to be replaced. If the head is in warranty, they will replace it. I thought I had a bad head once as well, similar to what you describe. Support talked me through some stuff and it turned out to be fine, so I did not need a new head.
HP still sent me some free ink anyway for the hassle routing tech support! First question for tech needs to be, "do you know what a z3100 is? have you ever seen one?" If the answer is "no" you have the wrong dept. and need to be asked to be routed to the Designjet folks.
Thanks. Unfortunately I'm going to be going out and buying a new head tomorrow. I need work done NOW, not waiting even for overnight package to arrive from them. It's $56 where I bought my printer from and they are only 45 minutes from me (assuming Chicago traffic doesn't blow!)
I'll call them in the morning, but who knows.. maybe they will get it to work. I did notice the SMALLEST (I mean small) bump in the ribbon, maybe there is a small cut in the ribbon causing bad communications with the cartridge.
This all reminds me I need to save up money for the extended maint. package. I rather spend the $500/yr (I think it's $1500 for extra 3 years on site) than pay for a service call, which I thought someone said was at least $700 for them to come out.