The diffusion pump is used to achieve deep vacuum pressures during the second pass, and a healthy system will reach 0.08 mbar (8.0 -2 on the gauge) with just the roughing pump, and around 0.005 mbar (5.0 -3 on the gauge) with the diffusion pump.
First, verify that the system can reach 0.08 mbar with just the roughing pump, and if not, troubleshoot for vacuum leaks or roughing pump performance by following SOP-Vacuum Troubleshooting.
If the system can reach 0.08 mbar with just the roughing pump, typically a malfunctioning diffusion pump simply needs to be cleaned and refilled with fresh diffusion pump oil.
More information can be found in SOP – Diffusion Clean and SOP – Vacuum Troubleshooting.
If you need additional assistance, please call technical support at +1 (206) 452-1130.