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HVAC Diagnostic Guide HIGH URGENCY

Thermostat Says COOL ON But No Cooling

Thermostat displays active cooling icon/indicator, indoor blower runs, but no cold air comes out.

Quick Check Before You Call a Pro

  1. Walk outside — is the outdoor unit actually running (fan spinning, compressor humming)?
  2. Check copper lines at outdoor unit — small line should be cold to touch in cooling mode
  3. Measure temperature differential at supply vs return — should be 18-22°F
  4. If you have a heat pump, verify thermostat mode is COOL, not HEAT (stuck reversing valve symptom)

01 · Most Likely Causes (Ranked)

35%

Outdoor Unit Not Running

What's happening

Indoor blower runs (you feel air movement) but the outdoor compressor and fan are dead. Thermostat doesn't know the difference — it sent the cooling signal and shows 'cool on.' Could be failed capacitor, contactor, tripped breaker, or compressor failure.

How to check

Go outside. Listen for the unit. Should hear compressor hum + see fan spinning. Silent = problem.

How to fix

Diagnose outdoor unit: check breaker, disconnect, capacitor (most common), contactor. See `ac-not-turning-on` page for detailed troubleshooting.

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20%

Stuck Reversing Valve (Heat Pump Only)

What's happening

If you have a heat pump, the reversing valve may be stuck in heating mode. You called for COOL, indoor blower runs, outdoor unit runs, but it's pumping heat INTO the house instead of OUT.

How to check

Heat pump only. Feel the smaller insulated copper line at outdoor unit — should be COLD in cooling mode. If warm/hot, reversing valve is stuck.

How to fix

Sometimes tapping with a rubber mallet frees it. Otherwise, replacement by a tech (~$300-500 brazing job).

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15%

Severe Refrigerant Loss

What's happening

System has lost most of its refrigerant. Compressor runs but produces almost no cooling capacity. Different from minor low refrigerant — this is so empty it barely makes a difference.

How to check

System has been declining for weeks. Indoor coil access shows no condensation (normally wet). Low-pressure cutout switch may also trip the compressor on/off rapidly.

How to fix

Major leak repair. Tech finds source (may be obvious — oily residue), repairs, evacuates, recharges. $400-1,000 typical depending on leak location.

10%

Disconnected Indoor Coil

What's happening

Rare but possible: the refrigerant lines have come disconnected or have a major break BETWEEN the outdoor unit and indoor coil. Air handler blows air, outdoor unit runs, but the cold refrigerant never reaches the indoor coil.

How to check

Visible damage to copper lineset (kink, break, disconnect). Sometimes after major service or after settling.

How to fix

Tech repairs/reconnects lineset, vacuum, recharge. Major repair, $500-1,500.

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10%

Failed Compressor

What's happening

Compressor is electrically running (drawing current) but mechanically not pumping. Internal seal failure, broken connecting rod, or worn piston. Rare but does happen on older systems.

How to check

Outdoor unit fan runs but compressor sounds different (quieter than normal, or different tone). Suction line is room temperature instead of cold. Confirmed by tech with pressure gauges showing low compression.

How to fix

Compressor replacement rarely makes economic sense on aging systems. Plan for system replacement.

10%

Indoor Coil Blockage

What's happening

Severely dirty evaporator coil or completely frozen coil. Refrigerant flows but air can't exchange heat through the coil. Indoor blower runs but air comes through unchanged.

How to check

Pull air filter — heavily clogged? Open indoor unit access — visible ice or dirt? Has the system been running poorly for weeks?

How to fix

If frozen: shut off, fan on, thaw 3-6 hours, replace filter. If dirty: tech cleans coil with proper foam cleaner. After fix, identify and address root cause (filter neglect, low refrigerant, etc.).

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CapacitorsReversing ValvesLine SetsAir Filters

02 · Frequently Asked Questions

Why does my thermostat say cool on but no cold air?
Thermostat just shows whether it's TRYING to cool, not whether cooling is actually happening. Most common reason: outdoor unit isn't running (failed capacitor, tripped breaker, dead compressor). Walk outside and check first.
How can I tell if my outdoor AC unit is running?
Stand near the outdoor unit. Healthy operation: top fan spinning, compressor humming inside (steady tone, not chattering), large copper line getting hot, small copper line getting cold. Silent or only partial = problem.
Should I keep running the AC if it's not cooling?
No. Continued running without cooling means the compressor is working hard for no result — wears the motor, contactor, and capacitor unnecessarily. Shut off at thermostat until diagnosed.
What's the difference between 'AC not cooling' and 'thermostat says cool but no cool'?
'Not cooling' usually means weak/insufficient cooling. 'Cool on but no cool' means zero cooling — much more severe, usually indicates major equipment failure rather than gradual issue.

03 ·Related Symptoms

AC Not CoolingAC Not Turning OnAC Blowing Warm Air
⚠ Safety notice: This guide is for informational purposes only. HVAC systems involve high-voltage electricity, refrigerants under pressure, and natural gas — all of which can cause serious injury, death, or property damage. Refrigerant handling requires EPA Section 608 certification by federal law. When in doubt, contact a licensed HVAC professional.
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