"washing" it will not fix it. The diagnostic hints they provide are rather useless (turn on, then off, then on) and those instructions will never fix anything. The warning appears when the vacuum pump is unable to create enough vacuum to degas the mobile phase. IOW: It is broken or just worn out.
You need a properly running degasser to maintain reliable flow and calm baselines. The fault may be due to a broken vacuum pump and/or possibly contaminated or leaking vacuum chambers. The Thermo/Dionex vacuum degasser is mounted on its own special frame inside the pump module. It needs to be removed and sent in for professional service. They are not made to be fixed by end -users. It is critical to have it professionally repaired, and not just swap in parts. Example: If you find the vac pump has failed, then replace it with a new vac pump, that pump too will probably fail. The reason is that you did not find out what caused the pump to fail, so replacing it does not diagnose the problem, but instead delay the repair (and yo are out the money you spent).
These inline vac modules only last 4-5 year and are designed to be replaced or serviced at these intervals. The manufacturer can replace the degasser module for several thousand dollars and get you going. However, if you want to save money and have it serviced with the same exact parts for a fraction of the cost, then look at this link: http://www.chiralizer.com/hplc-degasser-repair.html
We repair these all of the time and turnaround time is just two days.