Ho many pounds of LOW pressure refrigerant does the 2000 toyota sienna take.

If you are talking about the pounds PRESSURE (PSI), it depends on the ambient temperature. That information is what is on the dual set of gages that you see refrigeration techs using. If you are talking about the weight of the refrigerant to fully charge an empty system, there should be a sticker under the hood, near the AC system that tell you the type and how much to charge the system, it will also tell you how much and type oil to add. That all pertains to a completely empty system. The amounts specified by the Toyota are very important.
Also, you know that an empty system has to have the system purged of air and moisture, That has to be done by pumping a vacuum on the system for about an hour. That usually provides a vacuum of about 22"hg. If you have replaced a major component or have disconnected any connections you will have to do that. Knowing exactly how much oil to put into the system is a challenge unless you know that there is no oil in there now. So it take some expertise and equipment to charge a system correctly.
ALSO, IT IS IMPORTANT THAT I TELL YOU AND THAT YOU KNOW THAT IT IS AGAINST THE LAW TO CONTINUAL RECHARGE AN AC SYSTEM THAT HAS A LEAK OF REFRIGERANT. THE LAW SAYS YOU HAVE TO CORRECT THE LEAK BEFORE YOU CHARGE THE SYSTEM.

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Gaker