Short version: I'm not sure there was ever a pressure issue but it's working now.
Long version:
I did borrow a gauge and checked the pressure, the gauge connections were not low loss, (found out about those after the fact), so I lost some pressure when connecting it but it was about where the 410A pressure chart said it should be for that outdoor temp, 103PSI at 33F. (However one HVAC tech I spoke to said you can't accurately tell charge level by pressure but I don't have the ability to pull the charge and weigh it.)
I added some more anyway up to about 110PSI and turned it on and it started working. Pumped it to about 120PSI while running in cooling mode, then switched it to heat and that worked, except I left the gauge hooked up and in heat mode the low pressure line becomes high pressure and pegged the gauge at 500 PSI. Switched it back to cooling to drop the pressure, then disconnected the gauge, which was a real disaster because the refrigerant came spraying out hard icing up the connector and making it hard to grip with pliers. I thought I lost everything but turned it on again and it's working. Soaped up the valve that had pressure under the cap and watched it for a while, no bubbles at all. (HVAC tech told me the valves could leak a bit when first opened up till the oil lubes the O rings, pressure under the cap could have been left over from initial pressurizing.) MrCool said if it's working with no errors then it's fine. I guess I put in a lot more refrigerant than it needed but lost just the right amount when disconnecting. Total cluster F but I have a working unit, no thanks to MrCool "support".
However, after I went through all that I did find a YouTube comment that said the P4 error might be fixed by disconnecting the compressor to control board plug and plugging it back in, which I had done earlier when checking compressor resistance, but never tried it after plugging it back in because I didn't think I had done anything. So it may have not been a refrigerant problem to begin with.
Long version:
I did borrow a gauge and checked the pressure, the gauge connections were not low loss, (found out about those after the fact), so I lost some pressure when connecting it but it was about where the 410A pressure chart said it should be for that outdoor temp, 103PSI at 33F. (However one HVAC tech I spoke to said you can't accurately tell charge level by pressure but I don't have the ability to pull the charge and weigh it.)
I added some more anyway up to about 110PSI and turned it on and it started working. Pumped it to about 120PSI while running in cooling mode, then switched it to heat and that worked, except I left the gauge hooked up and in heat mode the low pressure line becomes high pressure and pegged the gauge at 500 PSI. Switched it back to cooling to drop the pressure, then disconnected the gauge, which was a real disaster because the refrigerant came spraying out hard icing up the connector and making it hard to grip with pliers. I thought I lost everything but turned it on again and it's working. Soaped up the valve that had pressure under the cap and watched it for a while, no bubbles at all. (HVAC tech told me the valves could leak a bit when first opened up till the oil lubes the O rings, pressure under the cap could have been left over from initial pressurizing.) MrCool said if it's working with no errors then it's fine. I guess I put in a lot more refrigerant than it needed but lost just the right amount when disconnecting. Total cluster F but I have a working unit, no thanks to MrCool "support".
However, after I went through all that I did find a YouTube comment that said the P4 error might be fixed by disconnecting the compressor to control board plug and plugging it back in, which I had done earlier when checking compressor resistance, but never tried it after plugging it back in because I didn't think I had done anything. So it may have not been a refrigerant problem to begin with.
