First identify what is actually not working
A blower that does not move any air has different likely causes from airflow that stays cold. Note whether the engine reaches its normal operating temperature, whether the heater works only while driving or at higher rpm, and whether the temperature differs between the left and right sides of the cabin. An accurate description makes diagnosis much faster.
When airflow is weak or absent
Weak airflow can be caused by a clogged cabin filter, the blower, its speed controller or the air-distribution controls. If the blower does not respond to different speed settings, coolant temperature is usually not the main issue. Inspection starts with the filter, fuses, power supply and the operation of the blower and air flaps.
When air flows but stays cold
In a combustion-engine car, the heater uses heat from the engine coolant. Cold air can therefore be related to a low coolant level, trapped air, the thermostat, restricted flow through the heater core, the water pump or a temperature-mixing flap. The faulty part cannot be identified reliably without checking temperatures, pressure, flow and any stored control-unit faults.
What the engine temperature gauge can reveal
If the engine does not reach its usual temperature after normal driving and the heater remains weak, the thermostat may be stuck open. A temperature rising toward the red zone together with weak heating can indicate a more serious circulation problem or coolant leak. Follow the manual for the specific vehicle and never ignore an overheating warning.
When to stop safely
Stop safely and switch off the engine if a red cooling warning appears, the temperature rises quickly, coolant leaks, steam is visible, or a sweet smell is accompanied by misted windows or a wet carpet. Do not open the cap of a hot expansion tank. Continuing to drive can seriously damage the engine; if in doubt, arrange recovery.
How a workshop checks the fault
The inspection starts by confirming the symptom, coolant level and condition, engine operating temperature and blower operation. Depending on the result, it may continue with flap-control diagnostics, hose temperature measurements, a cooling-system pressure test or a heater-core flow check. At MotPro in Prague 9 we first identify the cause and agree the repair scope with you in advance.
Every vehicle and fault is different. This article provides general guidance; an inspection of the specific vehicle determines the correct procedure.