If your external air conditioning (AC) or heat pump unit is still running while your thermostat is shut off, the most probable explanation is that your compressor contactor, located in the external unit, is stuck. However, there is a chance you may be experiencing one of two other issues that can cause your external unit to continue running.
The content below orders the three potential causes by what is the most likely to what is the least likely.
While You Wait
Switching the breaker in your circuit board will shut your unit off while you await help. It is best to keep your system off, but you should note that this will not resolve the problem. The moment you start up your AC system again, the external unit will resume operating in excess.
1. Stuck Compressor Contactor
Simply put, if your external unit is functioning properly, a 24-volt current is sent to the compressor contactor when you turn the thermostat on “cool.” This current makes the contactor close, powering up both the compressor and the condenser fan motor. When the thermostat is turned off, the voltage no longer energizes the contactor, inevitably powering down the compressor and condenser.
Yet, when the contactor closes, there is an arcing, or curved trajectory, of electricity. It is a normal occurrence; however, it heats up the contactor and can, over time, make the contactor contacts (there are typically two) stick together or weld shut in a closed position. With the contactor being unable to perform as it should and disconnect, interrupting power, the condenser continues to operate, even when the thermostat is shut off.
For more on this, refer to this video to watch a professional swap out a welded contactor, and go here for a visual comparison of a fried contactor with a healthy one.
How to fix the problem: The contactor will have to be replaced by an HVAC technician. If you locate the contactor and it is stuck shut but not damaged, you might be able to unstick it by tapping on it. Using a small wire brush to clean it off might allow you to run the AC temporarily without the contactor sticking immediately. But again, that’s not a long-term solution, and it should be replaced.
Cost? DIY? According to our AC Repair Guide, the cost to replace a contactor is $125-$375 depending on part price, local cost of living and whether the repair is made during “normal” business hours or is an after-hours “emergency” repair. Most homeowners pay about $180 for the repair when hiring a pro.
The part, a single-pole, 24 volt, 40 FLA amp definite purpose contactor costs $10-$20. The repair might be suitable for experienced DIYers who know what safety measures must be taken when working with electrical components.
Here’s the part we’re talking about. Note the reviews from those who bought it - “A quick repair on an old AC unit and a bunch of money saved.” Or this one, “Worked on a Lennox XC14 5 Ton Condenser.” Other comments refer to it working on Trane, York and Goodman units. This is a universal part for a central AC or heat pump condensing unit.
Even if a contactors is not your problem this time, it might make sense to have it replaced preemptively or have a fresh one on hand to do the job yourself if needed later.
How to prevent the problem later: There is a way to dodge this issue in the future, but it is not cheap. The reason the contactor breaks down over time is because it is made out of copper, which is an inexpensive metal and a worse conductor than other, more costly metals. If you want a contactor with a long lifespan, consider purchasing a commercial-grade contactor with gold, silver or platinum contactors.. With these, you may have to pay more upfront, but it is possible you will never have to change the contactor again.
Otherwise, as a rule of thumb, it is necessary to replace the contactor every two to three years as preventative maintenance that might keep you from having an issue during really hot weather - or cold weather if it is a heat pump. Or simply replace it when it gets stuck.
2. Damaged Thermostat Wiring
You may find that some of the thermostat wiring connected to the condensing unit is damaged. One of the reasons for this may be that the wiring is simply old. Due to time and age, the protective insulation around the wiring deteriorates, exposing the copper wires themselves. Those exposed wires then touch and maintain a constant stream of electricity to the compressor contactor, keeping the condenser running and unable to shut off with the rest of the AC system.
Or, perhaps the wiring is not old. The same AC unit issue will happen if the connecting wires are damaged. There are a number of things that may cause wire damage, but here is a brief list of a few common ones:
You can inspect the indoor wiring by taking off the thermostat cover and removing the screws holding it to the wall. Outside, look for damage to the conduit housing the wiring and inspect any bare wires you can see.
Pro Tip: An easy way to deduce if the issue is your thermostat is by letting the condenser run while turning the power off to your air handler (that is, the blower). You can do this by flipping the switch on the air handler or furnace to the “off” position or by turning off the circuit on the electrical panel. Normally, with the power off, the compressor contactor will not receive the voltage it needs to keep running. If the condenser also turns off, then you will know the thermostat—and not the contactor—is the issue.
How to fix the problem: Because the situation is electrical, you should contact an HVAC technician to connect new wire from the thermostat to the condenser.
However, until a professional can diagnose your specific case, it will be difficult to determine if the core problem is the thermostat wiring or the thermostat itself. If you suspect the wiring is not the root issue, then refer to the “Defective Thermostat” section below.
How to prevent the problem later: Because there are a number of reasons the wiring can be damaged, there is no specific prevention method we can suggest. However, you can rest assured that most prevention methods are practical, such as building a fence around the external AC unit if you suspect a rodent or weed-whacker had damaged the outside wiring, or assuring that any nails being put in the wall are away from the thermostat.
3. Defective Thermostat
If you cut the power to your air handler and the condenser stops running, then your thermostat—not the thermostat wiring—is the problem. To be certain, carefully unplug the yellow wire from the Y terminal on your thermostat. The yellow wire is connected to the cooling system, if your thermostat was properly wired. If your unit shuts off, then your thermostat is defective.
How to fix the problem: You or an HVAC professional will need to replace your thermostat.
Contact an HVAC Professional
While DIY problem solving can save you money, these three specific issues are not DIY friendly to anyone without experience. As a result, you will need to contact your nearest or most reliable HVAC technician to repair your external unit or thermostat.
This kind of work usually comes with a warranty. For more information on the types of HVAC company warranties and what they cover, see the Pick HVAC Warranty Guide.
To understand the condition of your AC unit, we recommend arranging either two 6-month AC unit inspections or one yearly inspection with your HVAC technician. Doing so, you will likely be able to avoid these issues with your AC system or, at the least, catch the problems before too much damage can occur.