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Home / Symptoms / AC Blowing Hot Air

Four common causes for an AC blowing hot air in the vehicle and their related parts.

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Is Your Car's AC Only Blowing Hot Air?

The air conditioner is blowing hot air because of sensor issues in the evaporator and ambient air temperatures. However, it can also be a failure in the blend door motor, compressor relay, or HVAC(Heating, Ventilation, and Air Conditioning) control head.

Is your car feeling more like a sauna? If so, don't sweat it - try to find out what's wrong first. Plus, when your car air conditioner system starts blowing warm air, it's essential to address it immediately. Not only can a malfunctioning AC be a real drag on your comfort, but it can also be a safety hazard if unchecked.

Watch out: Don't let the situation get any hotter than it needs to be. Instead, ensure your vehicle stays in peak condition, keeping you cool and collected on the road. Remember - a minor issue can quickly snowball into a major headache if left unaddressed. So, avoid future troubles and enjoy a worry-free ride by investigating earlier.

Let's Get To The Bottom Of It!

Do you hear a clicking/knocking noise from the dashboard?

The blend door motor is a little machine that helps control the temperature inside your car. It's like a robot that moves a plastic gear back and forth based on what you tell it to do on the AC control panel. The problem is that the plastic gears inside the motor can get worn and stripped out over time. If that happens, you might hear a clicking or knocking inside your dashboard when adjusting the temperature. So basically, if you hear weird noises when you mess with the temperature controls in your car, the blend door motor might be worn out.

Is the AC not obeying the temperature/fan speed/setting you put on the panel?

The control head is part of your car's AC system with buttons and knobs to change the temperature and where the air comes out. It's kind of like the panel of the AC system. Over time, the little switches and connections inside the control head can get worn. When that happens, they begin causing problems. For example, you might set the controls to make the air come out of the vents, but instead, it comes out of the defroster or the floor. That's a sign that the control head failed. So basically, if your AC controls aren't doing what they're supposed to, it might be because the control head is broken.

Is the AC blowing only slightly cool air, even if set on "maximum?"

The evaporative temperature sender is a small part that helps control your car's temperature. Its job is to let the climate control module know the evaporator's temperature, prevent it from freezing, keeping the car's interior at the right temperature. However, if the temperature sender isn't working correctly and gives the wrong temperature readings, it can cause problems with your car's air conditioning system. The system might think that the evaporator is frozen and reacts by shutting off completely. This is why ensuring the evaporative temperature sender works correctly is essential.

Does the AC compressor remains off even if you activate the AC switch?

In many vehicles, the climate control module (sometimes called the body control module) turns the AC compressor on or off. It uses information from the control head, engine sensors, and other car parts to determine when to engage or disengage the compressor. The body control module uses drivers to send a voltage signal to activate the compressor. These signals tell the compressor clutch relay to turn on, engaging the compressor. However, if the drivers in the body control module stop working correctly, the system may seem like it's trying to turn on the AC compressor. Still, it won't be able to send the voltage signal needed to engage the compressor clutch relay.

Does the AC compressor stay constantly engaged?

The compressor relay is an essential part of the AC system. Its job is to carry the voltage to the AC compressor clutch to engage the magnetic clutch and start running. However, over time, the contacts inside the relay burn, causing it to malfunction. This can result in the relay staying engaged even when no command voltage goes to the AC compressor. Consequently, the compressor continues to receive voltage even when it shouldn't, causing problems with the AC system.

Is the AC sometimes randomly switching from cold to warm when blowing air?

The Ambient Air Temperature Sensor stands outside the vehicle, in front of the radiator. Its main job is communicating with the car's computer so it understands the driving conditions. This task is crucial because it affects the engine's efficiency and power and the car's climate control. For example, the sensor tells the climate control system the outside air temperature, helping to decide how hard it needs to work to maintain a comfortable temperature inside the car. However, if the Ambient Air Temperature Sensor malfunctions, it tricks the climate control system into thinking it doesn't need to work as hard as it should. This can cause problems with the car's climate control and make it more challenging to maintain a comfortable temperature inside the vehicle.

The most common causes for a vehicle’s AC blowing hot air

The most common causes for the car’s AC blowing hot air:

Faulty blend door motor: The blend door motor, known as the blend door actuator adjusts the position of an opening called the “blend door,” which manages the airflow inside the cabin. The blend door motor works depending on what the driver sets the AC to; so it opens up the blend door a lot if the driver wants it very cold, for example. However, if the blend door motor fails, the AC might not work properly and can even blow hot air due to stress.
Faulty HVAC control head: The control head is a module that processes all the settings the driver makes about the AC. So, when the driver puts in that they want the coldest temperature, the module understands that and makes the internal arrangements for it to happen. However, if the module is faulty this “communication’ can get caught up, even leading to hot air coming from the AC.
Faulty evaporator temperature sensor: This sensor monitors the air passing through the evaporator, which is the component cooling the air before it enters the cabin. However, if this sensor is faulty, it might mistakenly report that the air coming from the vents is cold, when in fact it might be hot, leading to malfunctions in the AC.
Faulty compressor relay: The AC compressor is an electrical component that pressurizes refrigerant fluid, vaporizing it and cooling down the airflow in the AC. The compressor relies on relays to manage the electrical current needed to make it work. However, if the relay is faulty, the compressor lacks electrical power, resulting in AC issues, including hot air.
Faulty ambient air temperature sensor: The ambient air temperature sensor measures the temperature of the air outside the vehicle and tells it to the HVAC system. This sensor is vital for automatic climate control systems, where the HVAC reads the outside temperature and either cools down or heat up the cabin based on the readings.

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Frequently Asked Questions

Do you hear a clicking/knocking noise from the dashboard?

The blend door motor is a little machine that helps control the temperature inside your car. It's like a robot that moves a plastic gear back and forth based on what you tell it to do on the AC control panel. The problem is that the plastic gears inside the motor can get worn and stripped out over time. If that happens, you might hear a clicking or knocking inside your dashboard when adjusting the temperature. So basically, if you hear weird noises when you mess with the temperature controls in your car, the blend door motor might be worn out.
Is the AC not obeying the temperature/fan speed/setting you put on the panel?

The control head is part of your car's AC system with buttons and knobs to change the temperature and where the air comes out. It's kind of like the panel of the AC system. Over time, the little switches and connections inside the control head can get worn. When that happens, they begin causing problems. For example, you might set the controls to make the air come out of the vents, but instead, it comes out of the defroster or the floor. That's a sign that the control head failed. So basically, if your AC controls aren't doing what they're supposed to, it might be because the control head is broken.
Is the AC blowing only slightly cool air, even if set on "maximum?"

The evaporative temperature sender is a small part that helps control your car's temperature. Its job is to let the climate control module know the evaporator's temperature, prevent it from freezing, keeping the car's interior at the right temperature. However, if the temperature sender isn't working correctly and gives the wrong temperature readings, it can cause problems with your car's air conditioning system. The system might think that the evaporator is frozen and reacts by shutting off completely. This is why ensuring the evaporative temperature sender works correctly is essential.
Does the AC compressor remains off even if you activate the AC switch?

In many vehicles, the climate control module (sometimes called the body control module) turns the AC compressor on or off. It uses information from the control head, engine sensors, and other car parts to determine when to engage or disengage the compressor. The body control module uses drivers to send a voltage signal to activate the compressor. These signals tell the compressor clutch relay to turn on, engaging the compressor. However, if the drivers in the body control module stop working correctly, the system may seem like it's trying to turn on the AC compressor. Still, it won't be able to send the voltage signal needed to engage the compressor clutch relay.
Does the AC compressor stay constantly engaged?

The compressor relay is an essential part of the AC system. Its job is to carry the voltage to the AC compressor clutch to engage the magnetic clutch and start running. However, over time, the contacts inside the relay burn, causing it to malfunction. This can result in the relay staying engaged even when no command voltage goes to the AC compressor. Consequently, the compressor continues to receive voltage even when it shouldn't, causing problems with the AC system.
Is the AC sometimes randomly switching from cold to warm when blowing air?

The Ambient Air Temperature Sensor stands outside the vehicle, in front of the radiator. Its main job is communicating with the car's computer so it understands the driving conditions. This task is crucial because it affects the engine's efficiency and power and the car's climate control. For example, the sensor tells the climate control system the outside air temperature, helping to decide how hard it needs to work to maintain a comfortable temperature inside the car. However, if the Ambient Air Temperature Sensor malfunctions, it tricks the climate control system into thinking it doesn't need to work as hard as it should. This can cause problems with the car's climate control and make it more challenging to maintain a comfortable temperature inside the vehicle.

Typical fixes to address the cause(s)

These are typical processes and repair services that fix a check engine light issue

The most common cause: Faulty evaporator temperature sensor

The most common reason for a car’s AC blowing hot air is due to a faulty evaporator temperature sensor. The evaporator is the component that cools the air before it enters the cabin; this component relies on a sensor to know the actual temperature of the air passing through so it can make its calculations to either heat up or cool down the air. Therefore, if the sensor is sending wrong information, the evaporator might mistakenly heat up the air coming from the AC, leading to malfunctions.

Symptoms it causes: Apart from the AC blowing hot air you might notice humming sounds, AC intermittently working, heating problems, etc.
Low risk of priority: A malfunctioning evaporator leaves you with a faulty AC. So, if you value comfort inside the car and chill temperatures, you might consider this a higher priority.

How exactly does the evaporator temperature sensor go faulty?

The temperature sensor is made to last for much of a vehicle’s lifetime. However, as with any other car component, it can wear out over time. Additionally, this component might also suffer from electrical issues related to the battery, alternator, drive belt, etc. Also, physical stress like driving through bumpy roads constantly or getting into an accident can result in enough impact to misplace or damage the sensor.

Go safe: Ask for a mechanic to inspect your vehicle’s HVAC system to investigate if the AC evaporator temperature sensor is faulty due to surrounding components and if there’s further damage to check out. Many shops do transparent vehicle inspections and lay out your options after investigating your vehicle’s conditions.

What’s a vehicle inspection? It’s “detective work” on your vehicle, checking its systems to see if anything prevents the car from working as expected. In this case, mechanics examine different parts of your vehicle to track if anything is causing the AC evaporator temperature sensor to wear out. They’ll also check the condition of components during their inspection, noting which are causing the problem and which got affected by it, recommending to replace them in order of priority.

Transparency example in a faulty AC evaporator service

An HVAC system inspection is the most transparent process to understand why your vehicle’s evaporator temperature sensor is worn. This is because many components surrounding the evaporator, compressor, and blend door motor can have a say in the issue, indicating the real culprits that caused it all.

Let’s read an example of how this kind of inspection helps to service a faulty evaporator temperature sensor.

A customer dropped off their vehicle, reporting that their air conditioning appeared to work well during the spring season. However, now that the temperature has increased, the air conditioning system only blows warm air.

During the test drive, the technician noticed that the AC was warm and the check engine light was on. However, the technician found nothing related to the customer's concern during the vehicle health inspection. To investigate further, a vehicle code scan was performed, and it revealed an AC evaporator temperature sensor circuit low (P0537 code).

After receiving authorization for specialized procedures, the technician used a vehicle-specific scanner to check the climate control module's live data. The technician found that the evaporator temperature sensor was reading -40F, indicating a malfunction.

The -40F reading is an extreme deviation from the usual 32F to 40F degrees, indicating a sensor malfunction.

The technician then accessed the temperature sender in the airbox and verified that the wiring was intact, so it wasn't a wiring issue. Unfortunately, this confirmed that the evaporative temperature sensor was failing, resulting in the AC system's malfunction.

Faulty evaporator temperature sensor

In this case, the mechanic put the “Immediate Action” tag on the evaporator temperature sensor.

Similar Symptoms

Below are just a few examples of typical Symptoms and Fixes your car might be experiencing

Four common causes for an AC not cooling the vehicle and their related parts.