AC On But Room Not Cooling: Quick Diagnostics

Quick Diagnostics When Your AC Is Running But Room Won't Cool

You can hear your AC running, maybe even feel air coming from the vents, but the room stays uncomfortably warm. This frustrating situation has several possible causes, and systematic diagnosis helps you identify and solve the problem quickly.

Step-by-Step Diagnostic Process

Step 1: Check Vent Airflow Temperature

The first test tells you a lot about what's happening:

1. Place your hand directly over a supply vent
2. Feel the air temperature

Results and Meaning:

What You Feel	Likely Problem	Next Step
Cold air, weak flow	Airflow restriction	Check filter and vents
Room-temp air	Compressor not cooling	Check outdoor unit
Slightly cool air	Low refrigerant or dirty coils	Professional needed
Strong cold air	Sizing or distribution issue	Check room factors

Step 2: Inspect the Outdoor Unit

Go outside and check the condenser unit:

Is the fan running?

• Yes: Feel for hot air blowing out the top—this is normal
• No: Check circuit breaker; fan motor may have failed

Is the compressor running?

• Listen for humming from the large component (compressor)
• If fan runs but no compressor hum, compressor may be off or failed
• If you hear clicking but no startup, likely capacitor issue

Check for obstructions:

• Clear any debris (leaves, grass, trash) from around unit
• Ensure at least 2 feet of clearance on all sides
• Look for bent or damaged fins

Step 3: Verify Thermostat Settings

Common thermostat issues that prevent cooling:

• Mode: Must be on "cool" not "fan" or "heat"
• Temperature: Set at least 3°C below current room temperature
• Fan setting: Try "auto" instead of "on" for better cooling cycles
• Batteries: Dead batteries can cause erratic behavior

Step 4: Check the Air Filter

A clogged filter is the most common cause of cooling problems:

1. Locate and remove the filter
2. Hold it up to light—can you see through it?
3. If filter looks gray, matted, or blocked, clean or replace immediately
4. Run AC for 30 minutes after filter change and retest

Step 5: Inspect for Ice

Look for ice formation on:

• Refrigerant lines (copper pipes going to outdoor unit)
• Evaporator coils (inside the indoor unit)
• Outdoor unit components

If you find ice:

1. Turn off cooling immediately
2. Switch to fan-only mode to help melt ice
3. Wait 2-4 hours for complete thaw
4. Check and clean filter
5. Restart AC—if ice returns, call professional (likely refrigerant issue)

Step 6: Check All Vents

Walk through the entire home:

• Ensure all supply vents are open
• Move furniture blocking vents
• Check return air vents aren't blocked
• Look for disconnected flexible ducts in accessible areas

Room-Specific Issues

If Only One Room Won't Cool

• Check that room's vent: Open? Unblocked? Air flowing?
• Ductwork issue: Duct may be disconnected or damaged
• Sun exposure: Heavy sun on that room may overwhelm cooling
• Far from AC: Distant rooms may get less airflow

If Upper Floor Won't Cool

• Heat rises—upper floors naturally 2-4°C warmer
• Check that upper floor vents are fully open
• Consider partially closing lower floor vents to redirect air upstairs
• Use ceiling fans to improve air circulation

Common Causes Explained

Undersized AC

If your AC has always struggled to cool:

• AC may be too small for the space
• Use our BTU calculator to verify correct size
• Symptoms worse on hot days (can't keep up with heat gain)
• Solutions: supplement with additional unit, improve insulation, or upgrade AC

Low Refrigerant

Refrigerant doesn't get "used up"—low levels mean a leak:

• Gradual cooling loss over weeks or months
• Ice on refrigerant lines
• Hissing sound near indoor unit
• Requires professional diagnosis and repair

Dirty Evaporator or Condenser Coils

• Dirt insulates coils, reducing heat transfer
• AC works but cooling capacity reduced
• Condenser coils (outdoor) can be cleaned with garden hose
• Evaporator coils (indoor) often require professional cleaning

Compressor Issues

• Compressor is the heart of the system
• Symptoms: outdoor fan runs but no cooling, clicking sounds, high electric bills
• Common causes: capacitor failure, motor burnout, electrical problems
• Usually requires professional repair or replacement

Temperature Differential Test

Professional technicians use this test:

1. Measure air temperature at a supply vent
2. Measure air temperature at the return vent
3. Calculate the difference

Results interpretation:

• 15-20°C difference: Normal operation
• 10-15°C difference: Possible issue, may need service
• Less than 10°C: Definite problem, call technician

When to Call a Professional

Immediate call needed:

• Burning smell from AC
• Ice that keeps reforming
• Hissing or bubbling sounds
• Outdoor unit not running at all
• Circuit breaker keeps tripping

Schedule service soon:

• Cooling reduced despite clean filter
• Temperature differential less than 15°C
• AC runs constantly without reaching set temp
• Any issue you can't diagnose

Conclusion

Systematic diagnosis—checking vents, outdoor unit, thermostat, filter, and looking for ice—helps you quickly identify why your AC isn't cooling. Many issues like dirty filters or thermostat settings are easy DIY fixes. For refrigerant, compressor, or persistent problems, professional diagnosis ensures proper repair.