No heat or cooling for the only system in the house. System was low on Freon and locked out on low pressure switch. Inspected evaporator coil, outside condenser using electronic leak detector but couldn’t find any leak. Added Freon (410a), to bring pressure back to normal and system worked properly, in heating and cooling. Tested system in heating and cooling mode and let it run until reach thermostat temperature, with no issues. Told customer that if it happens again, will need to do an isolated leak search, to determine where the leak is located.