AC runs constantly
Never shutting offis two different faults.
One is a system that cannot stop. The other is a system that cannot catch up. They look identical from the hallway, they have nothing in common underneath, and one question separates them.
The one question that splits this
Look at your thermostat. Is the room temperature it displays at or below the number you set it to? That answer sends you down two completely different roads.
Yes — it has reached setpoint
The system cannot stop
The house is at temperature and the air conditioner is still running. Nothing is wrong with its ability to cool — something is preventing it from switching off, or continuing to tell it to run.
What it means: a controls or electrical fault. Frequently cheap, occasionally free, and one version of it needs calling about today.
Covered belowNo — it never gets there
The system cannot catch up
It runs and runs and the house stays two, three or five degrees above where you set it. The controls are working perfectly — they are calling for cooling because cooling has not happened yet.
What it means: a capacity or load problem. Could be the equipment, could be your ducts, could be that it is 108 degrees outside and this is normal.
Also covered belowIf you cannot tell, use the overnight test
A hot afternoon makes both faults look the same. A cool Fresno night separates them cleanly, and it costs you nothing but a glance before coffee.
1
Before bed, write down two numbers
The temperature you have set, and the room temperature the thermostat is currently reading. Leave the fan on AUTO and do not change anything else.
2
Check again at around five or six in the morning
This is when the outdoor temperature is at its lowest and the load on your system is a fraction of what it was at four in the afternoon.
3
Note whether the outdoor unit is still running
Step outside briefly, or listen for it. What matters is whether the compressor is running, not whether you can hear the indoor blower.
4
Read the result
Still running with the house at or below setpoint on a cool morning means a controls fault. Still running with the house above setpoint after a whole cool night points at capacity, refrigerant or airflow.
Why this works: almost no correctly functioning system in Fresno needs to run continuously at 5am in July. If yours does and the house is already cold, the equipment is fine and something in the controls is holding it on.
When it cannot stop: controls and electrical
The house is at temperature and it keeps running anyway. Costs are typical Fresno ranges rather than quotes.
The thermostat fan is set to ON
FreeThe most common explanation and the easiest to rule out. On ON, the indoor blower runs continuously whether or not the system is cooling, so you hear air moving all day and assume the air conditioner never stops. Switch it to AUTO and see whether the noise stops between cycles. If the outdoor unit was already cycling normally, that was the whole thing.
A welded or stuck contactor
$150 – $300The serious one. Contactor contacts can fuse closed, which means line voltage never disconnects from the compressor and the outdoor unit physically cannot stop, regardless of what the thermostat says. Covered in more detail below because it needs acting on rather than scheduling around.
A thermostat wiring fault
Quoted after diagnosisA short between the power and cooling wires produces a permanent call for cooling. Rodent damage to low-voltage wiring in attics and crawlspaces is not unusual here, and a staple driven through a cable during past work will do the same thing years later.
A failed or badly placed thermostat
$175 – $500A thermostat that has failed calling for cooling, or one reading several degrees warm because of where it sits, will hold the system on indefinitely. A thermostat in a hot hallway will run a comfortable house into the ground trying to satisfy a reading nowhere else in the building shares. More on thermostat faults.
A setpoint the system can never reach
FreeWorth checking honestly. A thermostat set to 65 on a 108-degree Fresno afternoon is asking for something no residential system is designed to deliver, so it will simply run forever trying. Setting it lower does not cool faster — a thermostat is a target, not a throttle.
A control board or relay fault
$400 – $900A relay stuck closed on the board keeps the call energized. Less common than the causes above, and most boards flash a diagnostic code that a technician should read before anything is replaced.
Call today, not next week
A stuck contactor means it runs until something fails
When contactor contacts weld themselves closed, the compressor is permanently connected to power. The thermostat can be off. The house can be freezing. It makes no difference — nothing in the control circuit can interrupt it, because the interruption point is fused shut.
That means a compressor running twenty-four hours a day, in Fresno, in summer, with no rest periods at all. It is the fastest way we know of to wear out a component worth thousands, and the only thing that stops it is switching off the breaker.
If your outdoor unit does not stop when you turn the thermostat off entirely, that is what you are looking at. Switch off the outdoor disconnect or breaker and call.
How to confirm it in one minute
- Set the thermostat to OFF entirely. Not up, not to fan-only. Fully off.
- Wait five minutes, then go outside and listen to the condenser.
- Still running? The contactor is stuck. Switch off the outdoor disconnect or breaker.
- Then call us rather than leaving it running while you think about it. The part is inexpensive; what it is currently doing is not.
When it cannot catch up: capacity and load
The system is trying and the house is not getting there. These are the usual reasons, and one of them is not a fault at all.
Low refrigerant from a leak
Reduced capacity means longer and longer run times before eventually never satisfying at all. Cooling weakens gradually over weeks, which is what distinguishes it from a sudden failure. More on leak detection.
A condenser coil packed with valley dust
The system cannot reject heat outside, so capacity drops and run times stretch. One of the most common and most preventable causes in Fresno. What a tune-up covers.
Restricted airflow indoors
A blocked filter, dirty evaporator coil or crushed ducting caps how much cooling can reach the house regardless of how well the equipment is working.
Duct leakage into a hot attic
Fresno attics pass 150 degrees in summer. Ducts leaking up there lose a meaningful share of your cooling before it reaches a room, and the system runs continuously trying to make up the difference.
A failing compressor
A compressor losing efficiency still runs and still cools, just less and less. Long run times with weakening performance are one of the ways this shows up. More on compressors.
Extreme heat, and nothing wrong
Systems are sized to a design temperature rather than the record high. On a genuinely extreme afternoon a healthy correctly sized system runs continuously by design, and that is not a fault. More on this.
If the air coming from your vents is not clearly cold, start with the AC not cooling guide instead — that covers the same causes from the other direction and has five checks you can run yourself.
Worth knowing
You are paying for this in real time
Unlike most air conditioning faults, this one bills you while you wait. A system running continuously when it should be cycling can add a meaningful amount to a PG&E summer bill, and the difference compounds every day it goes unaddressed.
Several of the causes on this page cost less to fix than a single month of the extra consumption. A stuck contactor is the clearest example: an inexpensive part, currently running your compressor around the clock and charging you for the privilege.
This is one of the few faults where waiting a month genuinely costs more than the repair.
Common questions
Why does my AC run constantly and never shut off?
There are two distinct explanations. Either the system has reached the set temperature and something is preventing it from switching off, which is a controls or electrical fault such as a stuck contactor, a thermostat wiring short, a failed thermostat or simply the fan set to ON. Or it has not reached the set temperature and is still trying, which is a capacity problem such as low refrigerant, a dirty condenser coil, restricted airflow, duct leakage or extreme outdoor heat. Checking whether the thermostat reading has reached the setpoint tells you which.
Is it bad for an AC to run all the time?
It depends why. Long run times on a genuinely hot day are healthy and easier on a compressor than frequent starting. Running continuously when the house is already at temperature is different, because the compressor never rests and you are paying for cooling you are not receiving. A system that runs all night in cool conditions is not doing something normal.
My AC will not turn off even with the thermostat off. What is that?
Almost certainly a contactor with welded contacts. The contactor is the switch that connects line voltage to the outdoor unit, and when its contacts fuse closed nothing in the control circuit can interrupt power, so the compressor runs regardless of the thermostat. Switch off the outdoor disconnect or breaker and call. The part is inexpensive at $150 to $300 in the Fresno area, but a compressor running around the clock is not.
Should my AC run constantly on a very hot day?
Yes, frequently. Systems are sized to a design temperature rather than the hottest day on record, so on an extreme Fresno afternoon a healthy correctly sized system may run without stopping and sit slightly above setpoint. That is the design working as intended. It becomes a fault when it also happens on mild days or overnight.
Does setting the thermostat lower make it cool faster?
No. A thermostat is a target, not a throttle. The system removes heat at exactly the same rate whether it is set to 72 or 62. Setting it very low simply guarantees it runs past the point you would have been comfortable, which is one way a perfectly healthy system ends up appearing to run constantly.
How much does it cost to fix an AC that runs constantly?
It depends on the cause. Switching the fan from ON to AUTO costs nothing. A contactor is $150 to $300, a thermostat $175 to $500, a control board $400 to $900 and a refrigerant leak repair $400 to $1,600. A dirty condenser coil is resolved during maintenance. See published repair pricing.
Will running constantly damage my air conditioner?
Continuous running is less damaging than frequent starting, since compressors wear on starts. The real costs are a compressor that never gets a rest period across a Fresno summer, and a meaningful increase in your electricity bill every day it continues. Several causes cost less to fix than a month of the extra consumption.
Tell us which version you have
Whether it has reached setpoint or not tells us where to start, and we measure the rest.
Guidance on this page is general information and not a substitute for a professional diagnosis. Prices shown are typical ranges for the Fresno area and are not quotes. Do not open the electrical panel on your outdoor unit — capacitors retain a charge after power is disconnected. California Heating & AC Repair is operated by Craftline Renovations, Inc. · CSLB License #999179 · Licensed and insured.