Common AC Cooling Diagnosis Reference

Why this matters

"AC isn't cooling" is the most common HVAC complaint in cooling season. Multiple possible causes; correct diagnosis matters for first-time fix. The tech who systematically diagnoses cooling complaints earns trust + saves time. This is the field card.

AC system basics

Components:

• Compressor (outdoor): pumps refrigerant
• Condenser coil (outdoor): releases heat to outside
• Condenser fan: moves air across condenser
• Expansion device (TXV / piston / orifice): meters refrigerant
• Evaporator coil (indoor): absorbs heat from inside
• Blower (indoor): moves air across evaporator
• Refrigerant: transfers heat between coils

Process:

• Liquid refrigerant evaporates in evaporator (absorbs heat from inside)
• Vapor compressed by compressor
• Vapor condenses in condenser (releases heat outside)
• Liquid returns to evaporator
• Cycle repeats

Common customer complaints

"AC not cooling at all":

• Compressor not running
• Power issue
• Capacitor failed
• Refrigerant lost completely
• Failure of major component

"AC running but not cool enough":

• Refrigerant low
• Dirty filter / coils
• Iced evaporator
• Compressor failing
• Undersized for conditions

"AC short-cycles":

• Thermostat issue
• Oversized for load
• Compressor overload
• Pressure issue

"AC freezes up":

• Low refrigerant
• Low airflow (dirty filter, blocked vents)
• Closed too many vents
• Thermostat set too low

"AC blows warm air":

• Outdoor unit not running
• Refrigerant low
• Indoor coil iced
• Compressor failed

"AC noisy":

• Fan blade damaged
• Bearing failing
• Compressor labored
• Refrigerant gurgling (low charge)

Diagnostic sequence

For "AC not cooling":

1. Verify customer setpoint (thermostat set correctly?)
2. Verify call for cool (thermostat showing cool calling?)
3. Check filter (most common; clogged restricts airflow)
4. Verify indoor unit running (blower? air at registers?)
5. Verify outdoor unit running (compressor humming? fan spinning?)
6. Check temperatures (return air vs supply air; 15-20°F delta normal)
7. Check refrigerant (gauges + manufacturer charge chart)
8. Identify root cause

Power diagnostics

No outdoor unit operation:

• Disconnect switch off (check at outdoor)
• Breaker tripped
• Contactor failed
• Capacitor failed
• 24V control signal missing (from thermostat / control)

Outdoor unit hums but won't start:

• Capacitor failed (most common)
• Compressor seized
• Locked rotor
• Hard-start kit needed (sometimes)

Indoor blower not running:

• Filter clogged (some systems shut down)
• Blower motor failed
• Capacitor (PSC motor) failed
• Limit switch tripped
• Float switch tripped (drain pan full)

Refrigerant diagnostics

Gauge readings:

Standard residential R-410A:

• Low side (suction): ~100-130 PSI (varies with outdoor temp)
• High side (discharge): ~250-400 PSI (varies with outdoor temp)

Low charge symptoms:

• Low side too low
• High side lower than expected
• Suction line cold OR icy
• Compressor short-cycling
• Insufficient cooling

High charge symptoms:

• Both pressures elevated
• Liquid back to compressor (slugging)
• Compressor failure risk

No charge:

• Both pressures low
• No cooling
• Often discoverable leak

Use manufacturer charge chart:

• Outdoor temp + indoor return temp = expected pressures
• Don't guess

Refrigerant leak detection

Electronic leak detector:

• Sniff fittings + components
• Most sensitive

Bubble solution:

• Soap solution on suspected fittings
• Bubbles = leak
• Simple + effective

UV dye:

• Added to refrigerant; black light reveals
• For hidden leaks

Common leak locations:

• Service valves
• Schrader cores
• Fitting connections
• Evaporator coil (most common in older systems)
• Condenser coil
• Compressor

Airflow diagnostics

Filter:

• Check first
• Replace if dirty
• Some systems have specific issues with high-MERV filters (too restrictive)

Ductwork:

• Verify supply registers open
• Check returns clear
• Check for collapsed flex duct

Blower:

• Wheel clean?
• Speed setting (multi-tap motor)
• Motor running correctly
• Capacitor (PSC) good?

External static pressure:

• Measure total static
• High SP = airflow restricted
• Low SP = duct leak

Evaporator icing

When evaporator ices over:

Symptoms:

• Reduced airflow
• Reduced cooling
• Eventually no airflow
• Sometimes blower water on floor

Causes:

• Low refrigerant (#1)
• Low airflow (filter, blower issue)
• Thermostat set too low (running constantly)
• Too many vents closed

Resolution:

• Shut off AC; let ice melt (turn fan on to speed)
• Don't restart until thawed
• Diagnose underlying cause
• Address before reoperating

Coil cleaning

Outdoor coil dirty:

• Reduces heat transfer
• Higher discharge pressure
• Less cooling
• Higher energy use

Cleaning:

• Hose from inside out (fins direction)
• Coil cleaner if heavy buildup
• Comb out bent fins
• LOTO during work

Indoor coil dirty:

• Often biological / dust buildup
• Cleaning more involved
• Sometimes pull from cabinet
• Cleaning chemicals available

Capacitor testing

Start capacitor (compressor):

• Common failure
• Bulged top = failed
• Test with multimeter (microfarads)
• Replace if outside ±10% of rated

Run capacitor (compressor):

• Allows compressor to run efficiently
• Failure: compressor draws more current
• Replace

Run capacitor (motor):

• For PSC motors
• Same approach

Contactor

Function: 24V signal from thermostat closes 240V to compressor + fan

Failure modes:

• Contacts welded closed (compressor runs constantly)
• Contacts pitted (intermittent operation)
• Coil failed (won't close)

Replacement: simple; verify same size + voltage.

TXV (Thermal Expansion Valve)

For systems with TXV (not piston / orifice):

TXV stuck open:

• Floods evaporator
• Low superheat
• Possible compressor damage

TXV stuck closed:

• Starves evaporator
• High superheat
• Reduced cooling

Diagnosis:

• Superheat measurement
• Sometimes replace if suspect

Compressor

Failed compressor (most expensive component):

• No cooling
• May hum
• High electrical draw
• Won't start

Diagnosis:

• Check capacitor first
• Check contactor
• Check incoming voltage
• Compressor windings test
• If failed: replace OR replace whole condenser

Drainage

Condensate drain clogged:

• Water in pan
• Float switch tripped (some systems)
• AC shuts off
• Sometimes water damage

Diagnosis:

• Inspect drain pan
• Check drain line for clog
• Clear with shop vac OR snake
• Flush + treat to prevent recurrence

Common shortcut traps

Adding refrigerant without finding leak:

• Solves problem temporarily
• Leak continues; problem returns
• EPA violation (knowingly adding to leaking system)
• Always find + repair leak

References

• ACCA Manual J + S + D
• EPA Section 608 (refrigerant)
• AHRI rating standards
• Manufacturer service manuals
• Manuall internal: Refrigerant Types Reference, Superheat + Subcool Charging