Outdoor AC unit keeps turning on and off. Why is that?

Picture this: You are sitting comfortably indoors, enjoying the cool breeze courtesy of your trusty air conditioner. Suddenly, you notice an unwelcome disruption in the system’s rhythm: the outdoor AC unit keeps turning on and off, leaving you puzzled and somewhat annoyed.

If you’ve experienced this frustrating scenario, you might know how much inconvenience it can cause.

Luckily for you, there’s a way for you to quickly troubleshoot and fix this problem as I am about to show you.

Outdoor AC unit keeps turning on and off

Often known as the condenser or the heat exchanger, this part is responsible for releasing the heat from your home’s air outside.

Now, before we proceed, let me first remind you that it is normal for an AC to turn on and off by itself.

This way, it is able to cool your house while being energy efficient.

The amount of time it takes between cycles is what makes the difference.

On average, an AC will run for 15 to 20 minutes before turning off for 10 or more minutes.

For the most part, this cycling time will depend on your thermostat settings and the current weather conditions.

That said, if you note that your AC is cycling more often than it should, it could be an indication that something is up.

So, what can you do about it?

Well, you can start by…

Adjusting the thermostat settings

Most ACs have their default cool differential temperature set to 0.5⁰F which is quite low. And, this might be causing the outdoor AC unit to turn on and off frequently.

Bearing that in mind, raise the cool differential to 1⁰F and work your way up according to your taste.

Other than that, you might opt to increase the compressor’s min (minimum) on time and the AC’s min cycle time.

Clean the coils and filters

You see, dirty coils and filters exert more pressure on the compressor. In turn, this forces the compressor to overwork and this will overheat it.

As a result, the compressor will turn off (to cool) before it resumes operation only for the cycle to repeat.

So, be sure to regularly clean the filters and coils.

And something else, make sure that there is no tall grass (or other obstructions) near the outside AC unit.

You see, such obstructions are likely to impede airflow near the condenser coils and, this will impede heat dissipation.

In turn, the refrigerant will flow to the compressor while it’s still hot thereby overworking the compressor.

Not to mention risk damaging it.

Clean the drain line

If the drain line is clogged, it will kick in the float switch once the line is full.

Once the line drains a little bit, it will deactivate the float switch (within seconds/minutes) only to reactivate it within a short while.

As you can see, this will toggle the AC on and off more frequently than it should.

So, be sure to also clean the drain line and remove any clogs that might be in there.

Visually check for reverse rotation of the outdoor fan

If the fan is rotating in the reverse and then stops within seconds or minutes, first disconnect the AC from the power.

Next, open up the unit and check the motor wiring to the control board, capacitor, and contactor.

Now, compare this with the wiring schematic on the service manual (or on the AC’s panel).

If the wiring is not properly done, be sure to connect the interchanged wires to their respective terminals.

On the other hand, if the wiring is good, proceed to test the capacitor (refer to the section below: test the capacitor).

Outdoor AC unit keeps turning on and off – additional troubleshooting steps

The evaporator’s thermistor might be faulty – so, test it

This thermistor senses the temperature on the evaporator coils and turns the outside AC unit on/off depending on this temp.

As you can see, if this thermistor is faulty, it won’t detect the right temp. As a result, it might continuously turn the AC on/off.

That being so, here’s how you can test this thermistor:

  1. While measuring the temp on the thermistor, use a multimeter to measure the resistance between the two wire connections.

At 20⁰C, the resistance should be around 6.1kΩ, and at 25⁰C, the resistance will drop to around 4.9kΩ.

Pro tip: These readings might vary from manufacturer to manufacturer. So, be sure to have your AC’s service manual nearby. Alternatively, you might check to see if your outdoor AC unit has a schematic diagram mounted inside its panel.

  1. Now, cover the thermistor with your hand. As the temp on the thermistor rises, its resistance should start to drop.

If the reading does not drop (or it doesn’t match the schematic’s readings), it means that the sensor is bad and should be replaced.

Test the capacitor(s)

To start with, if the capacitor is bloated, it’s going bad and it’s time to get a replacement capacitor.

Moving on…

As you might know, the capacitor is responsible for powering the motors and compressors.

Now, if the capacitor was to misbehave, it might result in the outdoor AC unit turning on and off within minutes (or seconds).

That being so, proceed to test the capacitance (in microfarads) on your capacitor(s) and compare it with the nameplate values.

Pro tip: Make sure to first disconnect the AC from the power supply and wait for the power to draw out from the capacitor. Otherwise, you risk getting electrocuted.

Now, if this reading does not match the nameplate value, it means that the capacitor is bad.

Fortunately, you can easily purchase and replace the capacitor with no special tools/skills.

Not to mention that they are cheap – see current AC capacitor prices.

Check if the contactor is dirty/bad

The contactor is another part that turns the outside AC unit on and off depending on the signal it receives from the thermostat.

Now, when the contactor fails, it might be responsible for continuously turning the outside AC unit on and off.

With that in mind, first turn off the power supply to the AC.

Next, access and remove the contactor.

Now, check if there are any bugs (or dirt) in there as they might be preventing the plunger from fully closing.

Other than that, check if the contactor is wearing out as this could also be resulting in the contactor malfunctioning.

Pro tip: A contactor that is fully gone will result in your AC not starting at all (or not stopping even if you turn off the thermostat). You can see how to test such a contactor in the video below:

And, if the contactor is bad, you can easily order a new contactor – they are pretty affordable.

Outdoor AC unit keeps turning on and off – other things to try

The refrigerant charge might be low – have it checked

In turn, this will overwork the compressor and subsequently, overheat it.

And as a result, the compressor will turn off (to cool) before it turns on again (and the cycle will repeat.

Unfortunately, it is not advisable for you to check the refrigerant as it is toxic.

Only an EPA-licensed HVAC technician can do this – so, you might consider hiring one.

P.S: If the refrigerant is low, the technician will recharge it (after finding and fixing the point it is leaking through).

AC compressor keeps turning on and off – what you can do

For the most part, this is due to the compressor overheating. And, in most cases, the fan is not affected.

Now, there are several possible causes for your AC compressor turning on and off.

And, each of these causes has its own troubleshooting steps.

Now, to learn how you can fix these problems, be sure to read our other article: Why is my AC compressor shutting off (and how to fix it).

It could be the control board

Most modern aircons have a PCB that acts as the ‘brain’ of the AC by controlling almost all AC functions.

Now, when this board acts up, it might result in the problems you are experiencing with your outdoor AC unit.

Bearing that in mind, start by checking the wire harness connections – confirm they are securely connected to the control board.

And while at it, confirm that none are frayed/burnt. If there is, be sure to fix/replace them.

Other than that, be on the check out for burn marks on the board. If you spot any, the board might be bad and you will need a replacement.

Luckily, you can easily order a genuine AC replacement control board for your model and replace it yourself.

Air conditioner starts then stops after only a few seconds

This is quite a worrying problem since the AC will never cool your room.

It might be due to several parts misbehaving/failing.

Now, when your AC starts then stops after only a few seconds, do the following:

  • Clean the drain line, coils, and filters.
  • Test the evaporator’s temperature sensor to confirm it’s working as it should.
  • Confirm the capacitance on the capacitor(s).
  • Confirm there are no bugs/dirt in the contactor.
  • Check for fan reverse rotation.
  • Check for refrigerant leaks.
  • Confirm that no outside air has found its way into the refrigerant line.
  • Confirm the compressor is not overheating.

P.S: The troubleshooting steps above should guide you along on how to fix these problems.

Outdoor AC unit keeps turning on and off – final thoughts

As we conclude, I have come across clients who are using ACs designed for large rooms in small houses.

This will result in the AC cooling the room pretty fast and turning off just as fast. And, this is not advisable as it might damage the compressor.

So, confirm that the AC you’re using is designed for a house of your size.


My air conditioner keeps turning on and off by itself (what you can do)

AC compressor overheating symptoms (and solutions)

Split AC compressor not working but fan is running [Fixed]

Window AC not blowing cold air but running [Fixed]

Leave a Comment