I had a problem with my z3200 and had the HP tech come out to fix out. Unfortunately, it didn't seem like he had any way of diagnosing the problem and he simply proceed to replace the circuit boards on printer. Basically, over the course of several visits he (1) updated firmware, (2) replaced formatter, (3) replaced "print-mech", and (4) replaced main pca board. Changing the pca board fixed the problem.
Swapping in the new circuit boards did not appear particularly difficult but the problem is there is no way to know what needs to be swapped out ahead of time. Also the hp tech did not have the parts on hand and had to make multiple visits.