Hey, Why Does My Car's AC Sometimes Work And Sometimes Don't?

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Four common causes for a vehicle A/C working inconsistently and their related parts.
The modern-day air conditioning system is computer-controlled. There is a multitude of sensors, switches, and motors that control the amount of cooling that the air conditioner puts out inside the vehicle. When these sensors start to act up, they can randomly affect the interior temperature. The interior temperature may fluctuate widely. Diagnosing these systems often requires the use of very dedicated specific equipment.

Is Your Car's AC Inconsistently Working?

It’s only working sometimes because the cabin air filter is too dirty, the “mode” door motor is faulty, or the variable compressor is stuck. Alternatively, the following parts might intermittently malfunction: the ac control unit, the ac high-pressure switch, and the evaporator temperature sensor.

Are you experiencing an unpredictable AC system in your car? One moment, it's working fine, and the next, it's not cooling at all? Air conditioners that blow cold air at one time and warm air at another are probably going through electrical issues.

Don't take this issue lightly! Ignoring it could put your safety at risk and lead to costly repairs. Instead, consider discussing with a professional to address the problem immediately.

Doing so ensures your vehicle remains in good working order, and you can drive it carefree, knowing your car air conditioner functions consistently. Don't let a minor AC issue become a significant problem that could have been avoided with timely maintenance.

Let's Get To The Bottom Of It!

  • Q: Is the car's AC air coming from somewhere apart from the vents?

    Your car's air conditioning system has several doors in its box that merges hot and cold air and direct it to where it must go. The Mode door motor is usually connected to a lever mechanism that controls multiple doors. However, if the Mode door motor gets miscalibrated, it causes air to come out of the wrong vent or not come out at all. This happens due to an intermittent connection that causes the motor to function only sometimes or stall, directing air to the wrong location.

  • Q: Is the temperature coming differently from opposite sides of the car?

    The AC control unit in your car is often built into the same panel as the fan and temperature knobs. Over time, the switch on this panel can wear out. When a switch wears out, it sends the wrong or no signal. This can cause problems for switch-controlled components, like the blower and air door control motors. For example, if one of the temperature switches is malfunctioning, it sends the wrong signal to the blend door on one side, causing the temperature on that side to be different.

  • Q: Is the temperature from the air coming out of the vents erratic?

    The AC high-pressure switch is essential to keep your car's air conditioning system running smoothly. It's responsible for monitoring the pressure on the "high" side of the system. If the high side pressure is out of range, the pressure switch signals to the climate control module about a problem with the AC system. If this happens, the compressor shuts off, meaning no cooling comes from the climate control system. Sometimes, however, the pressure switch can be erratic. This means it may shut the AC system off even if the pressure is within the normal range.

  • Q: Does your air conditioner stops for a long while before working again?

    The evaporative temperature sensor is a small but essential part of your car's air conditioning system. It's located in the airbox before the evaporator core, and it determines the core's temperature. This information is crucial for maintaining your AC system's efficiency. Another essential function of the temperature sensor is to alert the climate control module if the evaporator freezes over. If the sensor registers a temperature colder than the actual temperature of the evaporator, the climate control module thinks that the evaporator is frozen. This causes the AC system to shut down until the temperature rises.

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Vehicle Health Inspection Proof

Explore the following typical inspection results that show a potential cause for the symptom and select the one you believe is similar to your vehicle's issue.

Dirty cabin air filter

The proof image shows the restricted cabin air filter, highlighting the importance of regular maintenance and replacement of filters to ensure the efficient operation of the air conditioning system.

A customer brought their vehicle to the shop and mentioned the air conditioning was not cooling as usual. They thought the blower motor was wearing out because the air from the vents did not feel as sturdy.

During the test drive, the technician noticed the air conditioning was not cooling as expected, and the air from the vents was weak. So the technician conducted a vehicle health inspection to determine the cause of the issue. After checking the components, they found that the cabin air filter was clogged, reducing airflow.

The cabin air filter purifies the air before it goes through the evaporator core. However, over time, the filter clogs with dirt, mold, and debris, restricting the airflow and reducing the cooling performance.

The technician recommended replacing the cabin air filter to restore the airflow to normal. After replacing the filter, the technician checked the air conditioning system's performance. First, they used an AC machine to check the pressures in the air conditioning system and found that they were running in the normal range.

They also checked the temperature coming out of the center to vent with the AC on high and found that the vent temperature was at 38°, indicating that the AC was performing efficiently.

Does the issue look like this? if not accessible your shop will document it
Stuck variable compressor

The proof image shows the variable vane compressor intermittently sticking, causing the check engine light to come on.

A customer brought their vehicle and said the air conditioning system wasn't working correctly. Sometimes it would work well, but other times it wouldn't. The check engine light recently came on, and the air conditioner stopped working.

During the test drive, the technician noticed that the check engine light was on and the air conditioner wasn't cooling. They couldn't find anything wrong during the vehicle health inspection. They did a computer code scan and found a code indicating an issue with the variable AC compressor control circuit.

The technician got authorization to do additional diagnostics, which involved using a scanner to verify the code and check how the air conditioning system worked. Unfortunately, because of the code, the compressor was disabled, which means it wasn't working as it should.

Then, even after clearing the code, the compressor wasn't functioning correctly, and the code returned. The technician determined the compressor swash plate was stuck and that the compressor would need to be replaced.

Does the issue look like this? if not accessible your shop will document it
Faulty mode door motor

The proof image of the mode door motor and connector shows that they're intermittently malfunctioning.

A customer brought in their vehicle and reported having trouble with the air conditioning system. They claimed the AC went crazy and switched where the air came out. Sometimes the air would only come out of the defroster, whether they had the heat or the air conditioner on. Then randomly, the air would come out where it should.

During a test drive, the technician confirmed the air was coming from the defroster. They tried changing the mode from vent to floor to defrost multiple times until the air started coming out of the vents.

Moreover, during a vehicle health inspection, the technician did not notice anything about the customer's concern. As a result, additional diagnostics were authorized, and the technician used a specialized scanner designed for the customer's vehicle.

They tried to see if the control head sent the right signals to the mode door motor. After manipulating the control head and watching live data, the technician found that the control head was signaling correctly. They then accessed the mode door motor and used a lab scope to monitor the signal from the control head to the motor.

The technician found a signal from the control head, but the mode door motor only moved about half the time. This verified that the mode door motor has an intermittent open, which must be replaced.

Does the issue look like this? if not accessible your shop will document it
Intermittent ac control unit

The proof image shows the control head removed from the dash for testing. The control head controls the temperature and airflow within the car.

A customer brought in their car and complained the air from the driver's side was hot while the air from the passenger's side was cold.

During a test drive, the technician confirmed that the temperature on both sides of the car was low, even if the air conditioner was high. Moreover, the driver's side vent was blowing hot air while the passenger's side vent was blowing cold air.

After a vehicle health inspection, the technician noticed no issues relating to the customer's complaint. However, the customer authorized additional diagnostics tests to identify the problem's root.

The technician used a vehicle-specific scanner to identify if the control head was sending proper signals. While adjusting the temperature on the passenger's side, the technician observed the blend door motor and live data to see if it moved correctly, which it was. However, when adjusting the temperature on the driver's side, the motor did not move.

Because the blend door was in a difficult location to access, the technician had to remove the control head from the back. Then, with a lab scope, the technician examined whether the command from the control head was being sent to the blend door motor on the driver's side. The technician found that there was no signal from the control head.

Therefore, the control head was deemed defective and would need to be replaced.

Does the issue look like this? if not accessible your shop will document it
Intermittent ac high-pressure switch

The proof image shows the location of the high-pressure cutout switch, which was the source of the problem.

A customer reported that their vehicle's air conditioner stopped working randomly. The issue is unrelated to temperature or duration, as the times when it stops working vary.

During a test drive, the technician did not notice anything about the customer's concern. They also did not note any issues during the vehicle health inspection.

The customer authorized additional diagnostics, and the technician used an AC machine to check the AC pressures. When the technician started the vehicle and turned on the air conditioning, the temperature from the center vent was normal, and the AC system pressures were normal.

Moreover, after running the air conditioner for about 35 minutes, the compressor shut off. The pressures were still standard, but the technician found that the high-pressure switch was reading over 400 PSI on the scanner, and the AC control module was commanding the AC compressor to be off.

To verify the issue, the technician unplugged the AC pressure switch and ran a jumper across the switch, forcing a signal. This caused the AC compressor to turn on, confirming that the AC pressure switch was stuck and causing the air conditioner to shut down randomly. The switch was replaced to fix the issue.

Does the issue look like this? if not accessible your shop will document it
Intermittent evaporator temperature sensor

The proof image shows the location of the sensor. In this case, the dash was removed to access it.

A customer brought their car to the shop and said the air conditioner would work for a little while, then suddenly stop and restart after ten minutes.

During the test drive, the technician didn't notice anything wrong with the air conditioner. When checking the car during the vehicle health inspection, they saw nothing about the customer's complaint.

The technician then got authorization for additional diagnostics and hooked up the car to an air conditioning machine and a specific scanner for the vehicle. Next, they started the car and turned the air conditioner on high. Again, the pressures and temperature readings were expected, but the air from the center vent was only 48°, which is not as cold as it should be.

The technician compared the readings from the scanner and the air conditioning machine to the actual temperatures and pressures in the car and found that the evaporator core temperature was about 15° cooler than the thermometer reading.

They continued to run the car until the air conditioning compressor shut off, and at that point, the temperature sensor in the evaporator was reading 12°, while the air coming out of the center vent was still at 48°. This meant the evaporator temperature sensor was not working correctly and needed replacing.

Does the issue look like this? if not accessible your shop will document it

Typical Fixes to Address the Cause(s)

The following chapters bases themselves on experiences from our auto repair shop; we'll describe related problems' causes and fixes.

"Cabin air filter replacement" fixes "Dirty cabin air filter"

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Hey, Why Does My Car's AC Sometimes Work And Sometimes Don't?
Sometimes a problem is more challenging to describe than it initially looked like. If you are not sure your problem is described by this article, please find below similar vehicle symptoms, which might describe better the issue you are experiencing.

Other things your auto repair shop might talk about:

Car AC sometimes works sometimes it doesn't. Car air conditioning. Electric blower motor. Air compressor. Air blowing. car ac intermittent. expansion valve. air filters blow cool air