What is the principle of automobile heater?

I don't know what model you are, so it's hard to say.

There is a filter screen above the air inlet (external circulation air inlet) of the blower of a general automobile, so that foreign objects cannot enter.

However, the filter screen of some cars is designed on the side of the heater, and such cars may be stained with foreign objects.

Let's go to the 4S shop and check it carefully.