No Cool Diagnostic Checklist

🔌

Phase 1: Basic Power & Thermostat Verification

Verify thermostat calling for cooling

Set temp below room temp, mode on COOL, fan to AUTO, check display working
Check thermostat batteries (if applicable)

Replace batteries, wait for reboot, verify proper display and settings retained
Verify power at indoor unit

Check breaker ON, disconnect switch ON, measure 120VAC or 240VAC at unit
Verify power at outdoor unit

Check breaker ON, disconnect switch ON, measure 240VAC at unit

Expected: 240VAC ±10% (216-264VAC acceptable range)
Check 24VAC control voltage

Measure at transformer secondary, R to C terminals at thermostat

Expected: 24-28VAC | Low = bad transformer or short circuit
Inspect control board fuses

Check 3A or 5A fuses - replace if blown, investigate cause before powering up

🏠

Phase 2: Indoor Unit & Airflow Assessment

Check air filter condition

Dirty filter reduces airflow, causes icing, and reduces capacity - replace if restricted
Verify blower motor operation

Should run when calling for cool - check for proper speed, listen for noise
Inspect evaporator coil for ice/frost

Ice indicates low airflow or low refrigerant - must thaw before diagnostics
Check evaporator coil cleanliness

Dirty coil reduces heat transfer - inspect with flashlight, clean if needed
Verify all supply registers open

Check for closed dampers, blocked registers, crushed flex duct
Measure supply air temperature

Should be 15-22°F below return air temperature (typical split)

Target supply temp: 55-65°F | Return temp: 75-80°F (typical)
Check condensate drain operation

Verify drain clear, float switch not tripped, condensate pump working (if equipped)

❄️ IF EVAPORATOR COIL IS FROZEN

STOP: System must be thawed before continuing diagnostics
Thaw procedure: Turn off cooling, run fan only for 1-3 hours, or use defrost mode if available
Causes: Low airflow (dirty filter/coil), low refrigerant, metering device issue, blower failure

🌡️

Phase 3: Outdoor Unit Operation

Verify outdoor fan motor running

Check for 240VAC at motor, verify proper rotation direction (upward airflow)
Check condenser coil cleanliness

Dirty coil causes high head pressure - clean with coil cleaner and water
Verify compressor operation

Should run when calling for cool - listen for hum, feel for vibration, check amp draw
Check contactor operation

Verify 24VAC at coil, contacts pull in, 240VAC passes through to compressor/fan
Test run capacitor

Discharge safely, measure µF with meter - should be ±6% of rated value

Good: Within 6% of rating | Weak: 7-20% off | Failed: >20% off or shorted/open
Measure compressor amp draw

Compare to nameplate RLA (Rated Load Amps) and LRA (Locked Rotor Amps)

Normal: 80-100% of RLA | High: >RLA (check airflow/charge) | Low: <50% RLA (issue)
Check for outdoor obstructions

Verify 24" clearance all sides, no debris in unit, vegetation trimmed back

⚡ COMPRESSOR WON'T START

Hums but doesn't start: Bad capacitor, low voltage, mechanical seizure
No sound/no amp draw: No power, bad contactor, internal overload tripped
Clicking on/off: Overload cycling due to low voltage, bad cap, or compressor issue
Hard start kit: May help marginal compressor start in heat - not a permanent fix

❄️

Phase 4: Refrigerant Charge & System Diagnostics

Connect manifold gauges to service ports

Blue hose to suction (large line), red hose to liquid (small line), yellow to vacuum pump/tank
Record ambient outdoor temperature

Required for proper pressure/charge analysis - measure in shade near unit
Measure suction pressure (low side)

With system running 10+ minutes - compare to P-T chart for refrigerant type

R-410A typical: 115-135 PSI @ 75°F indoor
Measure discharge pressure (high side)

Should correlate with outdoor ambient temp - check P-T chart

R-410A typical: 250-300 PSI @ 30°C (86°F) outdoor
Measure suction line temperature

Use temp clamp or probe at service port on large line
Calculate superheat

Superheat = Suction line temp - Saturated temp (from P-T chart at suction pressure)

Target: 8-12°F typical TXV | 10-25°F typical fixed orifice (varies by system)
Measure liquid line temperature

Use temp clamp at service port on small line
Calculate subcooling

Subcooling = Saturated temp (from P-T chart at liquid pressure) - Liquid line temp

Target: 8-15°F typical (varies by system - check manufacturer specs)
Inspect for refrigerant leaks

Use electronic leak detector, soap bubbles, or UV dye at all joints/connections

📊 Quick Reference P-T Values (R-410A)

Suction @ 40°F evap

118 PSI

Suction @ 45°F evap

133 PSI

Discharge @ 80°F

225 PSI

Discharge @ 95°F

316 PSI

🔍 REFRIGERANT CHARGE DIAGNOSIS

Low superheat + Low subcooling: TXV stuck open, overcharge (rare)
High superheat + Low subcooling: Low refrigerant charge (most common)
High superheat + High subcooling: Restricted metering device or filter drier
Low superheat + High subcooling: Overcharge, restricted airflow, dirty condenser

⚡

Phase 5: Electrical System Diagnostics

Test contactor contacts

Check for pitting, burning, or welding - measure voltage drop across contacts (<0.5V)
Check compressor windings resistance

Power OFF - measure ohms: Common to Start, Common to Run, Start to Run

CS + CR should equal SR | Open circuit = failed winding | Low ohms = short
Test compressor to ground

Measure ohms from each terminal to ground - should be open circuit (OL)

Any continuity to ground = FAILED COMPRESSOR (do not operate)
Check dual run capacitor

Discharge safely, test HERM and FAN terminals - should match ratings ±6%
Verify proper wire sizing

Check conductor size matches breaker rating per code (typically 14AWG=15A, 12AWG=20A, 10AWG=30A)
Inspect all electrical connections

Look for burned wires, loose terminals, corrosion - tighten all connections
Test high/low pressure switches (if equipped)

Verify switches closed during normal operation - open on fault condition

💡 CAPACITOR TESTING

ALWAYS discharge: Use insulated screwdriver across terminals before handling
Visual inspection: Bulging top, oil leakage, burn marks = replace immediately
Testing: Must use capacitance meter (not multimeter ohms setting)
Replacement: Match or slightly higher µF rating OK, voltage must be equal or higher

🔬

Phase 6: Advanced System Analysis

Check metering device operation

TXV: Should adjust to load | Fixed orifice: Check for restrictions or ice
Measure airflow (if possible)

Target: 400 CFM per ton of cooling (e.g., 3-ton = 1200 CFM)

Use anemometer, flow hood, or temp rise method
Calculate system capacity

Use temp split and airflow: BTU/hr = 1.08 × CFM × ΔT
Check for non-condensables in system

High head pressure with low ambient = air in system (requires reclaim/recharge)
Test reversing valve (heat pump only)

Switch between heat and cool modes - feel lines shift hot/cold
Inspect compressor oil level/quality (if accessible)

Low oil or acid contamination indicates system failure
Document all measurements for report

Record pressures, temps, voltages, amps, superheat, subcooling, ambient conditions

🛠️ COMMON AC FAILURE MODES

Low refrigerant: High superheat, low subcooling, reduced capacity
Dirty coils: High head pressure, high amps, reduced capacity
Bad TXV: Erratic pressures, hunting, icing, or starved evaporator
Compressor failure: No pressure rise, low amps, short cycling
Restricted filter drier: High superheat, frost on drier, pressure drop across drier
Low airflow: Icing, high superheat, reduced capacity, high suction temp