"Hi Sir, I have a problem of my HP desigejet 5500 42', system self-test failure, and shows that system error: Air pressure system. How can I fix the problem and need change any parts? It is urgently and like to have your opinion with much appreciations. Best Regards Henry"
You will need to replace the air pressure system (APS). It is a small mechanism clipped under the ink supply station, fairly easy to replace. The replacement procedure is in the service manual which can be downloaded from MANUAL.
The part you need can be ordered here: http://www.lpscomputer.com/p-2507-c6090-60084-designjet-5000-5500-aps-air-pressure-system.aspx
Bob