Air conditioners that run but don't cool fail in predictable patterns. About 40% of these calls turn out to be capacitor failures (a $300 repair we can usually do same-visit), another 25% are refrigerant issues from slow leaks, 15% are dirty condenser coils restricting heat rejection, and the rest are split between contactor problems, thermostat issues, and frozen evaporator coils. The steps below get you most of the way to a diagnosis. The DIY portion of this is small but worth doing — sometimes it's just an obvious thing that\'s easy to miss.
Most common causes
The likely culprits, roughly in order of how often we see each one:
- Capacitor failed (most common)
The capacitor is the start/run component for the compressor and condenser fan. When it fails, the outdoor unit may hum but not actually run. Often you'll hear a click but no fan spin-up. Diagnostic visit + replacement: $280-$450.
- Low refrigerant from a slow leak
Refrigerant doesn't "get used up" — if you're low, there's a leak somewhere. Symptoms: weak airflow, ice on the indoor coil or refrigerant lines, AC running constantly without cooling. Top-up + leak diagnosis: $400-$700.
- Dirty condenser coil
The outdoor unit's coil sheds heat. When it's caked in cottonwood, grass clippings, or dust, heat can't escape efficiently and the AC can't cool the house. Visible from the side of the unit — fins should look clean.
- Frozen evaporator coil
The indoor coil ices over when there's low refrigerant or restricted airflow (clogged filter). Once iced, the AC can't cool. Turning the system off for 4-6 hours with the fan running thaws it, but the underlying cause needs fixing.
- Failed contactor
The contactor is the high-voltage switch in the outdoor unit. When it fails, the compressor and fan don't engage even though the thermostat is calling. Replacement: $300-$450.
- Thermostat or wiring issue
Sometimes the AC isn't getting the signal to run. Check thermostat batteries, mode setting, and that target temp is below current room temp.
- Compressor failure (worst case)
Compressor failure on a unit older than 8-10 years is usually a sign to replace the whole AC rather than repair. We'll be honest about the math.
What to check yourself first
Before calling, walk through these — sometimes the fix is something simple:
- Confirm the thermostat is set correctly
Mode = COOL, fan = AUTO, target temp 3-5°F below current room temp. If batteries are weak, replace them.
- Check the air filter
A clogged filter restricts airflow and is the #1 cause of frozen evaporator coils. Replace if the filter looks dirty.
- Check the breaker
AC has its own breaker (usually 30-60A double pole). If tripped, reset once. If it trips again immediately, stop — there's a real electrical issue.
- Look at the outdoor unit
Is the fan spinning? Is there debris (leaves, grass, plastic bags) blocking airflow? Does it sound like it's running normally? Hose down the outside of the coil if it's caked with debris (no pressure washer — gentle stream).
- Check for ice on the indoor coil or refrigerant lines
Ice = stop running the AC. Turn it off, set fan to ON to thaw the coil (4-6 hours), then call. Running an iced AC damages the compressor.
- Listen for the compressor
When the thermostat calls for cooling, you should hear the outdoor unit start up within 30 seconds (clicking contactor, then the compressor). If you hear humming with no fan spinning, that's usually a capacitor.
What NOT to do
- Don't add refrigerant yourself — illegal without certification, and it doesn't fix leaks
- Don't bend the condenser coil fins trying to "clean" them aggressively
- Don't keep running the AC when you see ice — you'll damage the compressor
When to call us
Stop troubleshooting and pick up the phone if:
- AC is running but indoor temperature isn't dropping after 30+ minutes
- You see ice on indoor coil or refrigerant lines
- Outdoor unit hums but the fan won't spin up
- AC breaker keeps tripping
- You hear unusual sounds (grinding, screeching, banging) from the outdoor unit
- Water is pooling near the indoor unit
- The AC has been running constantly all day in moderate weather and the house still isn't cool
Frequently asked questions
Why is my AC running constantly but the house isn't getting cool?
Usually one of three things: refrigerant is low (slow leak), the condenser coil is dirty (heat can't escape outside), or the AC is undersized for the home. Refrigerant and dirty coils are common; sizing issues are less common but real, particularly in homes that had additions or major window replacements after the original AC install.
Should I replace my AC if it's 12+ years old and not cooling?
Possibly. The repair-vs-replace math depends on the specific failure. Capacitor or contactor: cheap repairs, worth doing even on a 15-year-old unit. Compressor failure on an aging unit: replacement is almost always smarter — refrigerant migration during compressor swaps damages other components. We give honest assessments rather than pushing replacement.
Is it normal for my AC to ice up when it's humid?
No. Ice on the AC indicates either low refrigerant, restricted airflow (dirty filter, blocked vents), or running the AC when it's very cold outside. Turn it off, let it thaw 4-6 hours, replace the filter, and run again. If it ices up again, call for service.
How long does AC repair usually take?
Capacitor and contactor replacements are typically 30-60 minutes. Refrigerant top-ups with leak diagnosis: 60-90 minutes. Coil replacements: half a day. Compressor swaps: full day. Most repairs we resolve in a single visit.
Related services
If you need professional repair, the relevant services for this issue: