AC Running But Not Cooling? 8 Causes Ranked

Q: Why is my AC running but not cooling?

Eight common causes. Most likely: failed capacitor (25-30% of calls — outdoor unit hums, fan won't spin), refrigerant leak / low refrigerant (15-20% — gradual capacity loss), frozen evaporator coil (10-15% — ice visible on coil), dirty outdoor coil (10%), failed compressor (8-12% — most expensive), failed contactor (5-8%), thermostat malfunction (5-8%), or ductwork leak (3-5% — cold at coil but warm at registers). Diagnosis $89, repair $245-$895 for most; full system replacement $11,500+ if compressor failed on aging system.

Q: How do I know if my AC needs more refrigerant?

Symptoms include gradual capacity loss over weeks/months, ice buildup on indoor coil or refrigerant lines, longer run cycles trying to reach setpoint, higher-than-normal energy bills, and warm air at registers despite system running. Refrigerant doesn't get 'used up' — if it's low, there's a leak. Adding refrigerant without finding the leak is a temporary fix that fails within 1-3 years. Diagnosis includes UV dye + electronic leak detector to locate the leak. Total leak repair + recharge: $485-$895.

Q: Should I replace my AC if it's not cooling well?

Depends on age and specific failure. System under 8 years old with single component failure: repair makes sense. System 12+ years old with major failure (compressor, coil): replacement math usually wins, especially in LADWP territory with heat pump rebates available. Read our AC repair vs replace guide for the practitioner decision framework with rebate landscape and refrigerant phaseout factors.

Q: Why is my AC blowing warm air?

AC running, blower running, but air at registers feels room-temperature or warm — same problem as 'AC running but not cooling.' Check the 5-minute homeowner items first (thermostat mode, filter, registers, breaker, outdoor unit clear). If those pass, professional diagnosis required — most likely capacitor, refrigerant, or compressor issue.

Q: How long should I wait before calling for AC service?

Don't wait. Running AC with marginal cooling stresses compressor — what starts as $295 capacitor replacement can become $4,500 compressor replacement if you keep running it for days hoping it'll improve. If your AC is running but not cooling and the 5-minute homeowner check didn't resolve it, schedule diagnostic same-day during dispatch hours (8 AM-8 PM) or first thing next morning if calling overnight. Phones answered 24/7.

Q: Can dirty filters cause AC to not cool?

Yes. Heavily clogged filter restricts airflow across the evaporator coil, which causes coil temperature to drop below freezing, ice forms, ice blocks airflow further, system shuts off as safety. Cycle continues. Easy fix: replace filter, wait 4-24 hours for ice to thaw, AC works again. Hard lesson: filter restriction repeated over time damages compressor and blower motor. Change filters every 30-90 days.

Q: What's the most common AC problem in Los Angeles?

Capacitor failure — 25-30% of summer emergency calls. SoCal heat causes capacitors rated 10-15 years to fail in 4-7. Read our AC capacitor failure guide for full symptom list and replacement cost. Beyond capacitor: refrigerant leaks (15-20%), frozen coils (10-15%), and dirty outdoor coils (10%) round out the top causes.

Troubleshooting

AC running but not cooling is the second most common emergency call we run in SoCal summers. Outdoor unit running, indoor blower running, but air coming out of vents is room-temperature or warm. Eight different problems can cause this — some you can troubleshoot yourself in 5 minutes, others require professional diagnosis. This guide walks through the diagnostic decision tree we use on actual service calls, ordered by probability of cause. CSLB #1138898 (C-20).

Two quick forks first. If the unit won’t power on at all — no hum, no blower, nothing — that’s a no-start problem rather than a cooling one, so start at AC not turning on. If you’d rather follow a plain top-to-bottom homeowner walkthrough than a ranked decision tree, see why is my AC not blowing cold air. This page ranks the eight causes by likelihood.

Quick 5-minute homeowner check (do this first)

Before calling for service, verify:

Thermostat set to "Cool" mode and below current room temperature (sounds obvious — about 8% of "AC not cooling" calls are this)
Air filter clean — pull it out and look. Heavily clogged = restricted airflow = warm air at registers. Replace if dirty.
All registers and return vents open — closed bedroom doors with closed registers can simulate "AC not cooling" in main living areas
Outdoor unit clear of debris — leaves, plant overgrowth, snow/ice (mountain areas), small animal damage
Circuit breaker for AC system NOT tripped — check both indoor air handler breaker and outdoor disconnect

If all 5 pass, proceed to professional diagnosis. The 5-minute check resolves 12–15% of "AC not cooling" calls in our experience.

8 causes ranked by probability

Cause 1: Failed capacitor (25–30% of calls)

Symptom: outdoor unit humming, fan not spinning, OR fan spinning but compressor not running
Why it causes warm air: compressor isn’t running, no refrigerant cycling
Repair: capacitor replacement, $185–$295
See full guide: AC capacitor failure

Cause 2: Refrigerant leak / low refrigerant (15–20%)

Symptom: gradual cooling capacity loss, not sudden failure
Possible visible: ice buildup on indoor coil or refrigerant lines
Why it causes warm air: refrigerant is what removes heat. Not enough = no heat removal.
Diagnosis: leak detection (UV dye + electronic sniffer)
Repair: leak repair + recharge, $485–$895 (varies by leak location and refrigerant type)

Cause 3: Frozen evaporator coil (10–15%)

Symptom: ice visible on indoor coil or refrigerant line, water drip from indoor unit, AC eventually shuts off
Causes: low refrigerant, dirty filter restricting airflow, malfunctioning blower, dirty coil
Repair: thaw + diagnose underlying cause, $245–$485

Cause 4: Dirty outdoor coil (10%)

Symptom: AC running, less cool than usual, gradual decline
Why it causes warm air: outdoor coil can’t reject heat, system runs less efficiently
Repair: coil cleaning, $245–$385 (often part of tune-up)

Cause 5: Failed compressor (8–12%)

Symptom: outdoor unit attempting to start, breaker trips, OR running with reduced capacity
Most expensive failure type
Repair: compressor replacement $1,800–$4,500 OR full system replacement $11,500–$18,500
Decision often replacement for systems 12+ years old — see AC repair vs replace

Cause 6: Failed contactor (5–8%)

Symptom: outdoor unit not turning on at all, no humming
Compounding: pitting/burning visible on contacts when removed
Repair: contactor replacement, $285–$385

Cause 7: Thermostat malfunction (5–8%)

Symptom: thermostat shows correct temperature but cooling isn’t activating
Smart thermostats: WiFi/network issue blocking remote control
Repair: thermostat replacement or repair, $185–$685. See smart thermostat service.

Cause 8: Ductwork leak / disconnected duct (3–5%)

Symptom: cold air at coil but warm air at registers (especially upstairs/distant rooms)
Why: cold air escaping into attic before reaching rooms
Repair: Aeroseal duct sealing ($1,500–$4,000) or duct repair

Diagnostic decision tree

Quick decision tree for understanding:

Does outdoor unit make humming sound but fan not spinning? → likely capacitor
Is ice visible on indoor coil or refrigerant lines? → likely frozen coil (low refrigerant or airflow issue)
Is outdoor unit completely silent? → likely contactor or breaker
Is air at coil cold but air at registers warm? → ductwork issue
Is cooling gradually weaker over weeks? → likely refrigerant leak or dirty coil
Has the system been running constantly without cooling? → likely refrigerant or compressor issue

When to call vs DIY

DIY is reasonable for:

5-minute homeowner checks (filter, thermostat, breaker, registers)
Filter replacement
Outdoor unit basic clearing of debris

Call professional for:

Anything involving electrical components inside outdoor unit
Anything involving refrigerant
Compressor or capacitor diagnosis
Frozen coil that doesn’t thaw within 24 hours
Burning smells or visible smoke (turn off breaker first)

Real-world example

Pasadena, August 2025:

Customer: 8-year-old Lennox 13ACX 3-ton AC
Symptoms: AC running for 3 days, not cooling — house at 84°F
Diagnosis on site: refrigerant 28% low, leak detected at indoor evaporator coil U-bend
Customer options:
- Option A: leak repair + recharge, $885 (likely 1–2 year fix)
- Option B: full system replacement (system was 8 years old, mid-life)
Customer chose Option A — system has another 5–7 years of expected life
Total cost: $885 (diagnostic waived with repair)
Outcome: cooling restored within 4 hours of arrival
Follow-up: scheduled for refrigerant check in 18 months

Service area & dispatch

AC service across all 5 SoCal counties:

📞 West LA / Westside: (424) 766-1020
📞 Pasadena & SGV: (626) 499-5530
📞 Thousand Oaks / Ventura: (805) 977-9940
📞 Irvine / Orange County: (949) 785-5535
📞 San Bernardino: (909) 757-6455
📞 Riverside: (951) 744-9188

Phones answered 24/7. Truck dispatch 8 AM–8 PM same-day. After-hours scheduled for first dispatch the following morning. Related: AC repair, AC capacitor failure, AC short cycling, duct sealing, AC replacement, AC maintenance. Failure-mode repairs: refrigerant leak, fan not spinning, compressor replacement.

CSLB License C-20 #1138898 | Roman HVAC 777 LLC dba Venta Heating and Cooling

Frequently Asked Questions

Why is my AC running but not cooling? +

How do I know if my AC needs more refrigerant? +

Should I replace my AC if it's not cooling well? +

Why is my AC blowing warm air? +

How long should I wait before calling for AC service? +

Can dirty filters cause AC to not cool? +

What's the most common AC problem in Los Angeles? +

AC Running But Not Cooling? 8 Causes + Diagnostic Decision Tree