When an AC stops cooling, the most common culprits are a failed capacitor or contactor, or a clogged condensate drain line. Not a dead compressor. Not a system that needs replacement. In most cases, a trained technician can diagnose and fix the problem the same day.
What Causes an AC to Stop Cooling?
The two most frequent causes of an AC suddenly stopping are:
- A failed capacitor or contactor – small electrical components that help the system start and run
- A clogged condensate drain line – a blocked pipe that triggers a safety shutoff to prevent water damage
Both can make the system appear completely dead. Both are typically inexpensive fixes. Here is what you need to know about each.
What Is a Capacitor or Contactor, and Why Does It Cause My AC to Stop Working?
Your AC relies on two small electrical components to function:
- Capacitor – acts like a battery boost, giving the motor the surge of power it needs to start. Without it, the motor hums, strains, or will not start at all.
- Contactor – acts like a switch between your thermostat and the outdoor unit. When it fails, your indoor unit runs but the outdoor unit (where the actual cooling happens) shuts down entirely.
These components fail more often than anything else in an AC system, especially in North Houston, where systems run hard for months at a time in extreme heat. When either part fails, the whole system can look completely dead. If you are noticing other warning signs alongside this, see our guide on signs your air conditioning system needs service.
A technician can test both components in minutes. If that is the problem, you are back up and running the same day.
What Is a Clogged AC Drain Line and How Does It Shut Off My System?
Your AC does not just cool air. It removes humidity from it. That moisture drains out through a small pipe called the condensate drain line.
Over time, that line gets clogged. But it is usually not just dirt or algae doing it.
What’s Actually Clogging Your AC Drain Line?
The real culprit in most cases is something called Zooglea, a bacterial biofilm that forms white or gray slime inside warm, moist, dark spaces. Your condensate drain line is the perfect environment for it to grow.
Here is why it is especially common in North Houston:
- Communities like The Woodlands, Spring, Conroe, Tomball, Magnolia, and Montgomery sit in one of the most humid regions in the country. That humidity means your AC runs longer and pulls more moisture out of the air, which means more condensate flowing through that drain line and faster Zooglea growth compared to drier climates.
- The biofilm builds up slowly along the interior walls of the pipe. It is not a sudden event. It narrows the line a little more each week until flow stops completely, and homeowners never see it happening until the system shuts off or water shows up somewhere it should not.
- Zooglea does not just cause clogs. When it is present in your drain system, it can push musty odors into your home’s air supply. That is an indoor air quality issue, not just a mechanical one.
- A DIY vinegar flush can slow down surface buildup, but it will not clear biofilm coating the pipe walls. Only professional cleaning removes the full clog and the film causing it, which is one reason routine AC maintenance matters more than most homeowners realize.
When the drain line fully blocks, one of two things happens:
- A safety float switch detects the water backup and shuts the entire system off to prevent water damage, making it look like a major system failure
- The water overflows and leaks into your ceiling, walls, or flooring, turning a simple clog into a costly repair
The fix in most cases is a professional drain flush. Not a new system. Not a compressor replacement.
Should I Panic If My AC Stops Cooling in Summer?
No. A small component can cause a very big symptom. A failed capacitor, a tripped float switch, a Zooglea-clogged drain line – these are all fixable problems that are far more common than a compressor failure or full system replacement.
Jeremy Lawrence, owner of Air Woodlands and a North Houston HVAC veteran since 1978, puts it plainly: most AC emergencies turn out to be AC inconveniences once the right technician looks at them.
Get a proper diagnosis before drawing any conclusions, and before spending money you may not need to spend. For a deeper look at what to check before calling for service, see our full AC troubleshooting guide for The Woodlands.
Why Trust Air Woodlands for AC Diagnosis?
Air Woodlands has been diagnosing and repairing AC systems for families in The Woodlands, Spring, Conroe, Tomball, Magnolia, and Montgomery since 1978. That is 49 years of North Houston HVAC experience. With 500+ reviews and a 100% satisfaction guarantee, we are the locally owned team your neighbors call first.
Our residential AC services cover everything from same-day repairs to full system replacements, and we serve all of North Houston and The Woodlands area. Available 24 hours a day, 7 days a week, including nights, weekends, and the hottest days of the year.
Schedule AC Repair in The Woodlands or Surrounding Areas
If your AC stopped cooling, do not wait. Call us now at (281) 367-8135 or book your service online here. We will tell you exactly what is wrong and what it takes to fix it. No guessing, no pressure, no upsells you do not need.
Serving The Woodlands, Spring, Conroe, Tomball, Magnolia, and Montgomery, TX.
Frequently Asked Questions
Why did my AC stop blowing cold air?
The most common reasons are a failed capacitor or contactor (small electrical parts) or a clogged condensate drain line. Both can make the system appear completely dead but are usually same-day fixes.
Can a dirty drain line cause my AC to stop working?
Yes. A clogged condensate drain line triggers a safety float switch that shuts the entire system off to prevent water damage. It is one of the most common reasons an AC suddenly stops running.
What does a bad AC capacitor sound like?
You may hear a humming or clicking sound from the outdoor unit as the motor tries to start but cannot get enough power. Sometimes the system is completely silent.
How long does it take to fix a capacitor or contactor?
Most capacitor and contactor replacements take under an hour. If that is the issue, Air Woodlands technicians typically have you back up and running the same day.
Is it worth repairing an AC that suddenly stopped cooling?
In most cases, yes, especially if the system is under 12 to 15 years old. A failed capacitor or clogged drain line costs a fraction of a replacement. If financing ever becomes a consideration, Air Woodlands offers flexible financing options. Always get a proper diagnosis before assuming the worst.
Who should I call if my AC stops working in The Woodlands, TX?
Air Woodlands has served The Woodlands, Spring, Conroe, Tomball, Magnolia, and Montgomery since 1978. Call (281) 367-8135 24/7 or book online for same-day AC diagnosis and repair.
What is the white slime in my AC drain line?
That white or gray slime is called Zooglea, a bacterial biofilm that grows in the warm, moist environment of your condensate drain line. It is the most common cause of drain line clogs in humid climates like North Houston. A professional drain flush removes it completely. DIY vinegar treatments only address surface buildup.
