could i substitute 404a with 134a??
Why would you want to? 404 is a 502 replacement 134 is a 12 replacement. It may work but not well
You'd need a different metering device.
Might have to change oil.
Also, 134a isn't a real good gas for low temp aplications.
It runs in vacuum.
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How many times must one fix something before it is fixed?