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

asked by on November 20, 2015

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.

Was this answer helpful?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

loud buzzing on driver side while in motion. no dash lights on?

Hello, thank you for writing in. There are several moving parts that can cause a vibration in regards to the air conditioning system. You or a technician will have to go through the components one by one until the vibration...

Air conditioning makes whirring noise

Hi there. Depending on where the noise is occurring, this will help determine what is causing it. If the noise is coming from under the hood, then it is the compressor. It is common for the AC compressor shaft bearing...

AC blowing hot air

Hi there. It is possible that your may have a freon leak somewhere. More than likely it's leaking out of the compressor front seal. I would recommend having a technician come out and inspect the air conditioning system and determine...

Related articles

What Causes the AC System to Leak?
Your car’s air conditioner is responsible for keeping the interior cool and comfortable on hot days. However, in order to do that,...
How Can I Have the Vents Set to Cold Without the AC on?
The The modern automotive HVAC system offers a number of different capabilities to help keep drivers and passengers comfortable during hot or cold weather. There’s the air conditioning system, the heater, and the vent system (which doesn’t use either the...
How Can I Have the Vents Set to Cold Without the AC on?
The The modern automotive HVAC system offers a number of different capabilities to help keep drivers and passengers comfortable during hot or cold weather. There’s the air conditioning system, the heater, and the vent system (which doesn’t use either the...