📍 Quick Summary
- Air moving through your vents doesn’t mean cooling is happening. The blower circulates air independently of the compressor. If the compressor isn’t running, the air is not being cooled.
- Go outside and check the outdoor unit. Is the condenser fan spinning? Do you hear the compressor hum? No sound from the outdoor unit while the indoor fan runs = compressor or electrical fault.
- Touch the large insulated copper suction line where it enters the outdoor unit. It should be cold and sweating. If it’s warm or room temperature, the system is not cooling regardless of what’s running.
- Gradual cooling loss — worse each afternoon, better overnight — is almost always dirty condenser coil or low refrigerant, not equipment failure
- Sudden complete cooling loss — was working, now nothing — is usually a failed capacitor, tripped breaker, or contactor issue. Often a quick repair.
- Don’t run a system with ice on it — the frozen coil is insulating itself from the air and providing zero cooling while risking compressor damage
Component Activity Checker
Check each of the three main components and match the combination to the grid below. You can observe all three from inside and outside the house without opening any panels.
What Is Each Component Doing Right Now?
Indoor fan: feel airflow from vents. Outdoor fan: look through the top of the outdoor unit. Compressor: listen for a low hum or vibration from the outdoor unit — separate from the fan.
● Running normally
● Not running / weak
Indoor Fan
Outdoor Fan
Compressor
What It Means
Indoor Fan: ON
Outdoor Fan: OFF
Compressor: OFF
⚡ Electrical / Control
The outdoor unit is receiving no power or the contactor is open. Check the outdoor disconnect and circuit breakers first. A failed capacitor, blown fuse, or tripped breaker stops both outdoor components simultaneously. This is one of the most common and most fixable scenarios.
Indoor Fan: ON
Outdoor Fan: ON
Compressor: OFF/Quiet
⚙️ Compressor Fault
Fan runs but compressor doesn’t engage. The most common cause: failed run capacitor. The compressor may attempt to start (brief hum or click) then go quiet. Also possible: contactor closed but compressor internally failed, or compressor thermal overload tripped from overheating.
Indoor Fan: ON
Outdoor Fan: ON
Compressor: ON
💧 Refrigerant / Airflow
All components running but no cooling output. The refrigeration cycle is failing — not the electrical side. Low refrigerant charge, dirty condenser coil, frozen evaporator coil, severely clogged filter, or disconnected supply duct dumping cold air into the attic. Check the suction line temperature test below.
Indoor Fan: ON
Outdoor Fan: OFF
Compressor: ON
☀️ Condenser Fan Motor
Compressor running but condenser fan not spinning — or spinning slowly. Head pressure builds rapidly. The system may cool briefly then shut down on high-pressure trip. Condenser fan motor or its capacitor has failed. Do not run in this state — compressor overheating risk is high.
Indoor Fan: WEAK
Outdoor Fan: ON
Compressor: ON
🔌 Blower Failure / Ice
Weak indoor airflow with outdoor unit running — look for ice on the evaporator or suction line. A failing blower motor or capacitor, a severely clogged filter, or an iced-over coil reduces indoor air delivery dramatically. The system runs but can’t exchange heat. Shut off cooling and check the filter immediately.
Cause Detail by Scenario
Failed Run Capacitor Compressor
The most common cause of a running outdoor fan with a silent compressor. The capacitor provides the electrical boost the compressor needs to start and maintain rotation. When it fails, the compressor hums briefly at startup but cannot turn. A technician can test and replace a capacitor in minutes — and it’s one of the least expensive AC repairs. Capacitors are more likely to fail during extreme heat and after power surges.
Low Refrigerant Charge Refrigerant
When refrigerant is low from a leak, suction pressure drops and the evaporator coil cannot absorb adequate heat. The system runs but delivers progressively weaker cooling. The suction line will be warm or only slightly cool rather than cold and sweating. The correct repair is leak detection and repair first, then recharge — adding refrigerant without finding the leak will result in the same failure within weeks or months.
Dirty Condenser Coil Outdoor Unit
The condenser coil rejects heat to the outdoor air. A coil clogged with grass clippings, cottonwood, dust, or debris cannot release heat effectively. Head pressure rises, cooling capacity drops, and the compressor may overheat and trip its thermal overload. Gradual cooling loss that is worst in the afternoon and better overnight is the classic pattern. Annual coil cleaning prevents this entirely.
Disconnected Supply Duct Airflow
A supply duct disconnected in the attic dumps conditioned air directly into the attic or crawlspace. The system runs and cools — but the cooled air never reaches the living space. Clues: the system seems to run forever without reaching setpoint, some rooms are colder than others for no apparent reason, and the suction line test shows the refrigerant cycle is working. Attic inspection confirms the disconnected duct.
Compressor Thermal Overload Trip Compressor
The compressor’s internal thermal overload trips when it overheats — from a dirty condenser, weak capacitor, low refrigerant, or high ambient heat. The compressor shuts off and must cool before restarting — typically 20–45 minutes. The outdoor fan may keep running during this period. Cooling that works briefly in the morning but fails by afternoon is often a thermal overload pattern.
Gradual vs. Sudden Cooling Loss
Gradual loss — the AC cooled fine last month, worse this month, barely working now — is almost always dirty condenser coil or slowly leaking refrigerant. Both worsen over time. Sudden loss — was working yesterday, nothing today — is almost always electrical: failed capacitor, tripped breaker, or contactor issue. Sudden failures are usually faster and cheaper to fix than gradual ones, which have often caused secondary damage by the time they’re diagnosed.
Severity Classification
T.A.
From the Expert
"The suction line temperature test is the fastest triage step I have. Before I even open my tool bag, I touch that line. Cold and sweating — the refrigerant cycle is working and I’m looking at airflow or controls. Warm — the refrigerant cycle has failed and I’m pulling out gauges. That one piece of information redirects the whole diagnostic. The second thing I do is go outside and listen. Outdoor fan running, compressor quiet? I’m testing the capacitor immediately — it’s one of the most common AC repairs in summer heat and one of the quickest. A lot of homeowners have been told they need a new system when what they actually needed was a $40 capacitor. The key is not to keep restarting the system when the compressor isn’t engaging — each failed start attempt that hums and clicks and goes quiet is putting stress on a compressor that may already be struggling."
— T.A., NFPA CFI-1 · Licensed Electrician · OSHA 30
What You Can Safely Check vs. When to Call
✓ Homeowner-Accessible Checks
- Check and replace the air filter — always the first step
- Go outside: is the outdoor fan spinning? Do you hear the compressor running?
- Check the outdoor disconnect box and the circuit breakers for the AC
- Touch the suction line — it should be cold and condensing moisture
- Look for ice on the evaporator housing or suction line
- Note whether cooling loss is gradual (days/weeks) or sudden (overnight)
- Note whether cooling is better overnight and worse in the afternoon
- Inspect the outdoor unit for grass, debris, or cottonwood blocking the coil
✗ Professional Service Required
- Capacitor testing and replacement — capacitors retain high-voltage charge
- Refrigerant pressure testing and leak detection
- Compressor amperage testing — evaluates internal condition
- Contactor inspection and replacement
- Condenser coil cleaning — requires power disconnection and coil-safe chemicals
- Any scenario where the compressor attempts to start repeatedly without success
- Condenser fan motor or condenser fan capacitor replacement
Frequently Asked Questions
My AC was working fine, then suddenly stopped cooling overnight. What happened?▾
Sudden complete cooling loss — working one day, nothing the next — is almost always electrical rather than mechanical or refrigerant. The most likely causes in order of frequency: (1) Failed run capacitor. The capacitor can fail suddenly, especially after a period of high-stress operation. The outdoor fan may still run but the compressor won’t engage. (2) Tripped circuit breaker. Check the breaker panel — the AC breaker may have tripped. Reset it once. If it trips again immediately, there is an underlying fault that needs diagnosis before resetting further. (3) Failed contactor. The contactor is the high-voltage switch that connects power to the compressor and condenser fan. If its coil fails, the outdoor unit receives no power. (4) Outdoor disconnect fuse blown. Some outdoor disconnects use fuses rather than breakers — one blown fuse stops the outdoor unit completely. Any of these are diagnose-and-fix scenarios, not full system replacement scenarios.
Can a dirty air filter completely stop an AC from cooling?▾
Yes — in two ways. First, a severely clogged filter restricts airflow across the evaporator coil enough that warm indoor air can’t reach the coil in sufficient volume. The coil becomes too cold, eventually freezes over, and ice acts as insulation — providing zero cooling despite the system running. The system may blow cold air initially as residual coil temperature cools the limited airflow, then gradually warm air as the coil ices over. Second, a clogged filter causes the blower to work against high static pressure, reducing airflow even further and potentially causing the blower motor to overheat and derate or shut down. If your suction line is iced or if cooling loss began during a long cycle, replace the filter, switch to fan-only mode to thaw the coil (1–3 hours), then restart the system. If it cools normally afterward, the filter was the cause.
My AC cools fine in the morning but can’t keep up in the afternoon. Is that normal?▾
It’s not abnormal for a system to struggle somewhat more in the peak heat of an extreme afternoon — but significant comfort loss that only appears in the afternoon has two common causes. First, a dirty condenser coil: as the outdoor air temperature peaks in the afternoon, a dirty coil that couldn’t reject heat adequately in the morning begins failing more severely, often triggering compressor thermal overload trips. The system may completely stop cooling by 3–4 PM and recover overnight when temperatures drop. Second, compressor thermal overload: the compressor overheats under sustained afternoon load and trips its internal safety — then recovers overnight. If the afternoon cooling loss has been gradually getting worse over several weeks, a dirty condenser coil is the most likely cause and cleaning it may restore full performance. If the loss appeared suddenly last week with no gradual progression, a developing compressor or capacitor issue is more likely.
Key Takeaways
- Air moving through vents does not mean cooling is occurring. The blower, outdoor fan, and compressor are three independent components — observing all three narrows the fault category before a technician arrives.
- The suction line temperature test is the fastest triage available: cold and sweating = refrigerant cycle working; warm = refrigerant cycle failed. One touch, 30 seconds, major diagnostic value.
- Outdoor fan running, compressor silent = failed run capacitor in most cases. One of the most common, most fixable AC repairs. Do not keep restarting while this is the scenario.
- Gradual cooling loss (weeks) = dirty condenser coil or leaking refrigerant. Sudden cooling loss (overnight) = electrical fault, failed capacitor, or tripped breaker.
- If the system is cooling but can’t keep up in extreme afternoon heat, confirm the condenser coil is clean and the refrigerant charge is correct before drawing any conclusions about system capacity.
- Running a system with ice on the evaporator coil provides zero cooling and risks compressor damage from liquid refrigerant floodback. Switch to fan-only and thaw before restarting.