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According to the schematic/fault tree from Toyota, it looks like the AC will not operate if the engine is above the target coolant temp. In other words, if the vehicle is running hotter than it should because the electric water pump is not working correctly, the AC will not operate to avoid overheating the engine even more.
Are you having problems with the electric water pump?
Have you had the AC system checked to see if there is low refrigerant pressure?
I've heard really noisy coolant pumps in the prius when the coolant level is low. The pump has a hard time processing through air pockets and it whines like crazy. How does the coolant level look?
You could test the ac compressor by jumping the pressure switch. I attached a picture of its location. If you jump the connectors with a paperclip you can signal the system that there is enough Freon to run the compressor. Don't do it for very long, because if there isn't sufficient Freon it will damage the compressor since Freon acts as a lubricant for the compressor.
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