Can you mix R134A with 409A if not what will mix with 409A?

No, you should not mix R134A with R409A, as they are different refrigerants with distinct chemical properties. Mixing them can lead to system inefficiencies, reduced cooling performance, and potential damage to the refrigeration system. R409A is typically used as a drop-in replacement for R22, and compatible alternatives include R407C or R422D, depending on the application. Always consult the manufacturer's guidelines before mixing refrigerants.