This problem is a common problem of Honda Fit, and it will appear as long as the antifreeze is replaced. That's because the Fit car has no anti-freezing and deflation screws, and the position of the water tank cover is lower than that of the warm water tank, so the air in the warm water tank will exist inside and cannot be released.
I'll tell you the simplest way. Park your car on a slope of 15-30 degrees, with the front of the car facing up and the fan idling for 3-5 turns, which will basically exhaust the air in the warm water tank.
Note: Make sure there is enough antifreeze in the water tank.