Now clearly a 12 TXV on a 22 or 502 system will not work at all. The internal pressures of the valve will never overcome the evap pressure force and the evap will starve for refrigerant flow. Got it.

But with say; 22 and 502 - what would happen? The pressures are pretty close - that is; within the normal operating range of the valve. Once the SSH was set wouldn't the valve just carry on? Maybe hunt a little at times, but pretty much 'work'?

Which leads me to my present confrontation: An R-502 (well; purple power head label anyway <g>) flowing R-404 refrigerant.

Actually; I'm not even positive about That. It may well be a R-404 condensing unit piped to a R-502 evap, and then charged with R-22.

Hmmmmm . . . . how can I determine for certain What refrigerant is in the system? I mean without replacing it. <g>

Anyway, my original question is: will there be any substantial performance or reliability gain from replacing the 502 valve with a 404 valve?