Q: What Can Cause a Car Heater or an AC to Stop Working?

There are several parts to both the heating and AC system referred to as the HVAC (heating ventilation and air conditioning system) in your automobile. Each unit has a liquid or medium to transport heat, a fan to blow the cool or hot air into the passenger compartment, and a fan or fans and cooling units under the hood. Both use some sort of power or mechanical unit to circulate the fluids. Both systems are sealed systems. So the problem can be as simple as a blown or open fuse, defective blower motor, low refrigerant level in the AC system, just to name a few.

Typically, when considering an AC system that stopped working, we are more likely to be looking at a failure to control the temperature or have air circulate in the passenger compartment. A short in the fan circuit will effectively stop the operation of each of these systems. A defective AC compressor, low system charge, system overcharge or electrical failure of the control head, or in some cases with automatic temperature control (ATC) or the electronic control unit (ECU).

Additionally, defective cooling fans, a clogged condenser or evaporator, or even a restricted H valve or orifice can cause the AC to malfunction. Loss of the ability to cool the condenser under the hood at a stop due to a defective fan can make the system appear to stop as it seems the AC blows cold when moving but goes warm when stopping.

The main issue is to properly isolate whether the system is underperforming, or it stopped all together. When it comes to the heating system, low coolant level, especially due to a leak, can be the cause. Poor performance due to a bad thermostat or defective water pump can also be interpreted as the system stopping. In either case a stuck or malfunctioning mode door or heater control valve could be to blame as well. Lets not forget a slipping drive belt as a possible cause also.

So the critical factor here is the correct diagnosis of whether it is a poor performance of the system, or a total failure of operation. Even a blocked cabin air filter can restrict the flow of air and make it appear that the AC or heater system has failed. Over time this may also lead to a failure of the blower motor, which can then need to be replaced. The key here as i mentioned is to have the HVAC system properly diagnosed to get a quick and cost effective correction.