Will refrigerant 134 work in a system which had 409A?

No, refrigerant R-134a is not compatible with R-409A. R-409A is a blend that contains different components, and mixing refrigerants can lead to system inefficiencies, potential damage, or safety hazards. It is crucial to evacuate the system and ensure it is clean before charging it with the appropriate refrigerant. Always consult the manufacturer's guidelines for the correct refrigerant to use.