How is it that the car only blows cold air without hot air after replacing the hot air box?

1, the heater water tank is broken (yours is new, the chance is very small).

2, the water tank is blocked, you can go to the auto parts city, find a master who repairs the water tank to connect you with an air pump (there are two water pipes under the machine cover, remove them and connect them with an air pump).

You should first try whether there is hot air, whether the fan is running abnormally and whether the air duct is unobstructed.

Sometimes the antifreeze in the warm air pipe does not flow for a long time,

Condensation blocks the circulating pipeline.