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Hello, set the thermostat as low as it will go, 60F or so, and then tell me if the second stage compressor does not come on.
If it does not come on, wait until it warms to over 90F outside and try it again. So far I suspect that systems electronic controls are keeping the system in low stage mode be cause its not warm enough in the house or hot enough outside to support high stage cooling,
Do all that, let me know what you can discover, we can go from there.