First of all, you should rule out whether there is any gas in the heater water tank or water pipe. In other words, check to see if the antifreeze has been changed recently. If so, it means there is gas in the heater water tank. If your air conditioner doesn't feel as hot as it used to, it's possible that the heater pipe is blocked. You can also do an experiment, that is, keep the car stationary, boil the water tank, and then turn on the heater.