You need to be able to trust your car’s air conditioning on a hot day. The last thing you need is to press the button or turn the dial to MAX air and get ready to sign in relief only to have the cold air run out. You roll your eyes, wondering if you need to recharge the refrigerant, only to have it come back on again full blast.
No, it’s not that your car is possessed by evil spirits intent on making you sweat through your shirt. Your car’s AC is likely short-cycling due to a problem with the ambient temperature sensor, the AC compressor, the serpentine belt that powers the compressor, or an electrical issue.
There are a few other things that could be causing your car’s AC to turn on and off frequently or randomly. Unfortunately, none of them will get better on their own, and the longer you leave them, the larger the final repair bill will be.
To help you figure out what’s causing your AC to go crazy, we will have to pop the hood on and look closer at your car’s air conditioning components.
How Does a Car’s Air Conditioning System Work?
The beating heart of your car’s air conditioning system is the compressor. As the serpentine belt snakes its way around the engine, it passes over the AC compressor’s pulley, which helps drive refrigerant in its gaseous state through the lines to the condenser.
Here, the condenser draws heat out of it via a series of cooling finds and the radiator’s cooling fan. The refrigerant then starts to cool and reverts to its liquid state. It then moves through the lines through the drier and past an expansion valve, where it changes from high to low pressure.
The refrigerant fluid then passes through the evaporator, where a phase change causes it to boil inside the lines. The fan then blows the cold air chilled by the phase change in the evaporator into the cab. You sigh in relief as the cold air passes over you, and the now hot refrigerant is drawn back to the compressor to start the journey all over again.
What Is Short Cycling?
Short cycling in a car’s air conditioning system occurs when the system prematurely turns itself off. This might be due to a mechanical malfunction in the physical components of the AC system, or it might be something as simple as a wonky ambient temperature sensor.
The system shuts itself down, but your dash settings or the ambient temperature sensor recognize that your car’s interior is still boiling hot. So, the air conditioning system tries to start the cooling process all over again. The flaw that caused it to shut down is still going on, and it shuts itself down after a few minutes.
6 Reasons Your Car’s AC Short Cycles and How to Fix Them
It’s an infuriating cycle that will not be stopped by you turning dials, pressing buttons, and spitting out curse words at inanimate objects.
Below are some of the more common reasons why your car’s AC is short cycling or the potential faults that can be causing it to turn on and off.
1. A Bad Ambient Temperature Sensor
Newer vehicles have thermostats that read the car’s interior temperature and turn the AC on and off as needed. If the thermostat is bad or simply caked with dust, it won’t turn the AC on and off at the correct time.
If your car has an ambient temperature sensor, it’s likely in the headliner above the passenger or driver’s side seat. While you can’t access the sensor, most vehicles will let you reset it. All you have to do is press the AC, recirculate airflow buttons simultaneously, and hold them down for 5 to 10 seconds. This will reset the sensor, and the AC should stop turning on and off randomly.
If you have an older car, or your model doesn’t have an ambient temperature sensor, you’ll have to dig deeper to discover why your AC is short-cycling.
2. Debris Around the Condenser
Dead bugs, leaves, and other debris that makes it past the grill can clog around the AC condenser, causing it to struggle to dissipate heat. This manifests in your cab as a cool blast of air when the system starts up, but it gets tepid and might even turn off after a few minutes when it temporarily overheats.
Check the front of the engine bay and clear any debris from the condenser and/or the radiator’s cooling fan. If possible, give it a few blasts of compressed air. Then, let the entire system cool down to normal outdoor temperature and restart the AC to see if it runs normally again.
3. Clogged Lines or a Plugged Condenser
The material in the AC lines or the tubules inside the AC condenser can cause blockages, which hamper AC system performance, causing the system to turn off or short cycle. This usually happens in the condenser itself but can also happen in the lines between the compressor and the condenser.
The easiest way to find out if the condenser is clogged is to use an infrared thermometer to test the exterior temperature. However, you might have to remove your car’s grill to get at it.
Point the infrared thermometer at the inlet of the condenser and take a temperature reading. Then, compare it to the outlet reading on the lower opposite corner of the condenser. The temperature reading between the two should be dramatic in the neighborhood of 70 to 90 degrees difference.
If the temperature at both locations is the same, or within 20 degrees or so, then you either have a complete or partial blockage in the condenser or the lines leading between the compressor and the condenser.
Though here’s the really bad news. When there’s a clog in your car’s AC lines or the condenser, it’s usually caused by a broken-off piece of the compressor. This means your compressor is also in dire straits and might need to be replaced along with the condenser.
4. Low Refrigerant Levels
Low refrigerant levels in the car’s AC lines can cause the cooling effect to be sporadic. The leap in logic here is to buy one of those AC recharge kits and refresh the system. You’ve probably seen a buddy do it or heard your neighbor talk about how easy it was.
The problem is that your car’s AC lines are supposed to be a completely sealed system. It doesn’t burn refrigerant or consume it to make cold air magically blast out your vents.
So, if your refrigerant is low, that’s causing your car’s AC turn on and off, and you have a leak somewhere in the system. If you don’t find the leak, your fix will last a few days or a few weeks, and you’ll be right back to the same problem again.
It’s also worth noting that overloading your car’s AC system with refrigerant could severely damage the compressor and other components. So, it might seem like a tempting $20 fix, but if you don’t check the refrigerant levels and/or find the leak, you could just end up costing even more costly damage to the air conditioning system.
How to Test the Refrigerant Levels in Your Car AC System
Start by looking for a little window in the receiver/dryer area. Some cars have view ports that let you see the refrigerant in action. If you don’t see anything or a bunch of little bubbles, you should be suspicious that low refrigerant is causing your AC to turn on and off randomly.
To confirm the refrigerant levels, start by finding the low- and high-pressure ports of your AC system’s Schrader valves. You’ll need an AC pressure tester. Attach the gauges to the low-pressure service port and the high-pressure service port.
Then, start the car and set the air conditioning to its coldest setting. This is usually the MAX AC, or just turn the air conditioning on and select the recirculating feature. Let the car run with the AC on for a good 5 minutes to make sure the system is fully cycling and should be pouring cold air out of the vents.
Look at the pressure on the gauges. You should have a rating between 25 PSI to 45 PSI on the low-pressure port and a reading of 250 to 400 PSI on the high-pressure port. If both readings are below this range, you are likely low on refrigerant.
How to Find Leaks in AC Lines
If you don’t find where the refrigerant is leaking, you’ll end up with the same problem again in a few days or weeks. Along the way, you might compound the damage to the car’s AC components.
To find the leaks in the AC lines, you need to ensure there is at least a modest level of refrigerant in the system. This might mean partially recharging it if your pressure test indicates that it’s low or out.
While you might be able to spray soapy water all over your car’s AC lines, the easiest way to truly find all the potential leaks in the system is to use a refrigerant tester. It’s essentially an electronic nose that you can use to “Sniff” around the air conditioning system.
Once you find the source of the leak, you will know if it’s just a line that needs to be replaced or if a major component, like the condenser or dryer, needs to be entirely replaced by a mechanic.
5. Coil Freeze-Ups With a Clogged Cabin Air Filter
If your cabin air filter has sat around filthy and blocked up for too long, it can actually cause a freeze-up in the AC lines behind the dash, which causes the system to turn off after a while. You’ll probably also notice that the air from the vents isn’t as vigorous as a few months ago.
The cabin air filter is usually behind the glove box, and you’ll have to uninstall it to get at the filter and the other interior components of your car’s air conditioning system. If you aren’t sure where your cabin air filter is, check your owner’s manual or the repair manual specific to your make and model.
Pull the cabin air filter out and give it a good cleaning. Tap it to knock any physical debris out, and give it a few gentle blasts of canned or compressed air. If it’s very clogged, you might have to find a replacement.
The more significant concern is that the frozen lines could also be caused by moisture entering your car’s air conditioning system via a line or a component leak. So, if you clean the air filter and the air conditioning system turns on and off again randomly, then you need to test the refrigerant levels and look for leaks, just like we discussed earlier.
6. A Bad Compressor Relay
The air conditioning compressor has a relay and a magnetic clutch that engages when you turn the system on. It might turn on for a moment with a screeching or whining noise. Then disengage, turning the air conditioning system off again.
Figuring out if this is the gremlin at play starts with a simple eyes and ears test. Pop the hood and stand in the front of the car with the engine running. Have someone else activate the car’s AC system inside. You should hear some sort of click or buzz as the compressor clutch engages.
If it doesn’t turn on, or it whines/buzzes or clicks off again in a minute or two, then the AC compressor clutch is causing your air conditioning to turn on and off again. If it doesn’t turn on, the AC compressor clutch is dead.
Make sure to check the relay or fuse that runs the AC compressor. It’s usually in the engine bay fuse box. The air conditioning system won’t turn on if it’s completely burned out.
However, it could also be a problem with the car’s AC high-pressure or low-pressure switch that could prevent the air conditioning from running normally.
7. A Bad High or Low-Pressure Switch
A failing high or low-pressure switch is one of the more common reasons a car’s AC system might turn on and off randomly. There’s a sensor designed to monitor the pressure and state of the refrigerant as it passes through the high and lower-pressure sides of your car’s air conditioning system.
There is both a high-side pressure switch, as well as a low-side pressure switch. They act as safety devices and will disengage if there’s a problem to prevent damaging the compressor or other system components.
If you recently tried to recharge your AC refrigerant without testing the level or just juiced it up recklessly, the overly high-pressure condition in the lines could be the problem. In such a case, the pressure is at risk of damaging the compressor, and the high-pressure switch shuts the system down.
If you have a leak in the refrigerant lines or a partial clog somewhere in the system, like the condenser, the low-pressure switch can disengage the system. This might allow the air conditioning to run for a few minutes before shutting down when the sensor notes that it’s reached an intolerable level.
Both switches must be closed to complete an electrical circuit that runs to the car’s AC compressor. If either the low-pressure or high-pressure switch is open, it won’t complete the circuit. Without the necessary power, the AC compressor’s clutch doesn’t kick on, and the pulley from the serpentine belt won’t be able to power the system.
How to Test a High- or Low-Pressure Switch
The bush-fix way to test the high and low-pressure switches in your car’s air conditioning system is to bypass the switch with a little piece of wire in the electrical connections. This is basically forcing the switch to close and complete the circuit.
Test the high-pressure switch first with the wire inserted and the air conditioning system turned on. If the system turns off again or the AC compressor clutch disengages, then the assumption is that the low-pressure switch is failing. Repeat the same test bypassing the low-pressure switch. If the system runs normally, you’ll have found why the air conditioning keeps turning off and on.
From there, you’ll need to either find the leak in the system or find a way to bleed out the excess refrigerant that was pumped in accidentally.
8. A Dying Blower Fan
If the blower fan that delivers air from the heater core and AC vents is dying or shorting out, it could also cause cold air to blow out intermittently. This will often start out as weak-blowing air that is inconsistent with the setting you have the fan on. The rest of the car’s air conditioning system might be functioning perfectly fine, but the blower motor isn’t delivering cold air to the interior.
Blower motor fans have a limited lifespan; the more you run them, the more likely they are to burn out. It’s also possible for a short circuit in the wiring to affect the fan’s performance. The fan might run like normal, then you go over a big speed bump a little too fast, and it jostles the wire and breaks the electrical connection.
A dying blower motor fan can sometimes make a buzzing noise. You might hear a clicking sound if there’s debris, like a mouse nest caught at the edge of the fan blades.
To test your car’s blower motor, you’ll have to access it. You can usually get to it by removing your glove box or the lower portion of the driver’s side dash. If you’re unsure, the repair guide for your car’s make and model will have instructions for getting to it.
You can then test the blower motor with a multimeter set to measure Ohms. All you have to do is touch the probes to the terminals on the blower motor. If you get a reading of “Infinite,” the circuit is open, and your car’s blower motor is defective.
9. A Failing Blend Door Actuator
The ambient temperature sensor in your car’s interior relies on the blend door actuator to turn the air conditioning on and off when the target temperature is reached. If the blend door actuator gets jammed, it won’t respond properly to the system’s demands. This can cause the car’s air conditioning to blow cold air one moment and then seemingly turn off.
The problem with a blend door actuator will get progressively worse. Eventually, it will get jammed and stay jammed in one position. You’ll likely hear grinding or clicking noises as the little gears and plastic components struggle to move correctly as it worsen.
To find the blend door actuator, you will have to take off the glove box and possibly uninstall part of the lower passenger side dash. Some cars have multiple blend door actuators for heat, AC, and fresh and recirculated air. Either one could be at fault.
If it’s the hot/cold blend door actuator, the air conditioning will seem to come on and off randomly, with no cold air blowing out when the AC is turned on, regardless of the setting. If it’s the fresh and recirculated air blend door actuator, you will notice the AC going from vigorous to weak, but it would still probably give you a little cold air out of the vents.
Then, the same vents might be on the driver’s side and/or in the back of the car. Most cars with multiple climate control zones have one or both door actuators in each climate location! To find yours, you need a make and model-specific repair guide.
If you can access the blend door actuator, check it for physical damage, and it will likely need to be replaced.
Using a Code Reader to Diagnose a Bad Blend Door Actuator
Diagnosing a suspected blend door actuator might also be possible using the car’s OBD diagnostic port and a code reader set to monitor the system. You can then select the HVAC system report or the AC module report; the names and terms can vary by the automaker.
In the menu, you will find tests for “Blend Doors” and other HVAC performance information. You can then make changes to the console with the car running. One by one, switch each climate control zone between hot and cold and fresh and recirculate.
The change should appear in the panel display of the code reader or the app you pair the code reader to in a few seconds. If it doesn’t change in the car or the app, you’ll have zeroed in on which blend door actuator is failing and needs replacing.
No More Stop-and-Go Cooling
A car’s AC system that turns on and off randomly can be frustrating. While the easy temptation is to assume the refrigerant level is low, juicing it up without testing the system first can end up doing more harm than good.
Start by resetting the ambient temperature sensor inside the car. You can usually do this by holding the AC down and recirculating the air button for 10 seconds.
Then, visually inspect the compressor and condenser and clear away any debris. Have someone else turn the car’s AC on while you watch and listen to the engine for signs that the compressor’s magnetic clutch is engaging and staying engaged.
If it’s not, check the electrical relays as well as the high- and low-pressure switches. You can bypass each switch to eliminate the other.
If you still can’t find out why your car’s air conditioning turns on and off randomly, you’ll have to turn your investigation inside. Remove the glove box and parts of the lower dashboard to access the cabin air filter and the blower motor and perhaps glimpse the blend door actuator.
You can inspect and test each of these for faults. In a newer car, you might even be able to connect a code reader to the OBD diagnostic port. You can then test your climate control system performance by turning things on and off to see if the reader gives you the same information as the change you make to the dash controls.
Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.