AC Not Cooling? Complete Diagnosis Checklist for HVAC Technicians

Initial Assessment

When a customer reports their AC isn't cooling, a systematic approach saves time and ensures you don't miss anything. Start with these initial questions:

• When did the problem start?
• Did anything change recently (power outage, storm, work on the house)?
• Is it blowing air at all, or no air?
• Is the outdoor unit running?
• Are any unusual sounds or smells present?

Quick Visual Check

Before grabbing your tools, do a quick walk-around:

1. Check thermostat settings (mode, temperature, fan setting)
2. Look at the air filter
3. Check that supply and return vents are open
4. Observe the outdoor unit (running, frozen, damaged?)
5. Look for obvious issues (tripped breaker, disconnected wires)

Thermostat & Controls

Thermostat Issues

The thermostat is often overlooked but causes many "not cooling" complaints:

• Wrong mode: Set to heat, fan only, or off instead of cool
• Set point too high: Set above current room temperature
• Dead batteries: Many thermostats need battery replacement
• Poor location: Near heat source giving false readings
• Loose wiring: Check connections at thermostat and air handler

Testing the Thermostat

1. Set thermostat to cool, 5°F below room temperature
2. Listen for click indicating call for cooling
3. Verify 24V between R and Y terminals at air handler
4. If no 24V, check transformer and wiring

Airflow Problems

Restricted airflow is a leading cause of poor cooling and frozen coils.

Common Airflow Restrictions

Dirty Air Filter

The #1 cause of service calls. A clogged filter:

• Reduces airflow across the evaporator
• Causes the coil to freeze
• Makes the system work harder
• Can trip the high-pressure switch

Dirty Evaporator Coil

Even with regular filter changes, coils get dirty over time. Signs include:

• Reduced airflow despite clean filter
• Higher than normal superheat
• Visible dirt on coil surface
• Musty odors from the vents

Blower Motor Issues

• Faulty capacitor (motor hums but won't start)
• Bad motor (no operation at all)
• Wrong speed setting (multi-speed motors)
• Dirty blower wheel (reduced CFM)

Ductwork Problems

• Disconnected ducts in attic or crawlspace
• Crushed flexible duct
• Closed or blocked registers
• Leaky duct connections

Electrical Diagnosis

Power Supply Issues

1. Check disconnect at outdoor unit (fuses or breaker)
2. Verify 240V at contactor line side
3. Check indoor breaker and switch
4. Measure 24V at transformer secondary

Component Testing

Capacitor

Test as described in our capacitor troubleshooting guide. A weak or failed capacitor prevents motor startup.

Contactor

Check for pitted contacts, stuck armature, or coil failure. See our contactor guide for details.

Safety Controls

• High-pressure switch: Trips on high head pressure (dirty condenser, overcharge, restriction)
• Low-pressure switch: Trips on low suction pressure (low charge, restriction, low airflow)
• Compressor overload: Internal thermal protection

Refrigerant System

Signs of Low Refrigerant

• Warm air from supply vents
• Ice forming on refrigerant lines or evaporator
• Long run times without reaching set point
• Bubbles in sight glass (if equipped)
• Low suction pressure, low superheat

Checking the Charge

1. Connect gauges to service ports
2. Measure suction and discharge pressures
3. Calculate superheat and subcooling
4. Compare to manufacturer specifications

Finding Leaks

If refrigerant is low, always find and fix the leak before recharging:

• Electronic leak detector
• UV dye and blacklight
• Soap bubbles at suspected locations
• Common leak points: service valves, flare connections, evaporator coil, condenser coil

Compressor Issues

Compressor Not Running

If the condenser fan runs but the compressor doesn't:

1. Check for 240V at compressor terminals
2. Verify capacitor is good
3. Check compressor winding resistance (C-R, C-S, R-S)
4. Test for grounded windings (megohmmeter to ground)
5. Check for locked rotor (amp draw at startup)

Compressor Winding Resistance

For single-phase compressors:

• Measure resistance between all three terminals
• Lowest reading = R (run) to S (start)
• Highest reading = C (common) to opposite terminal
• R-S should equal C-R + C-S
• Infinite reading = open winding (failed)
• Zero/very low = shorted winding (failed)

Common Compressor Problems

• Mechanical failure: Internal damage, locked rotor
• Electrical failure: Burned windings, grounded
• Thermal overload: Temporary shutdown from overheating
• Refrigerant flooding: Liquid refrigerant washing out oil

Diagnosis Flowchart Summary

1. Thermostat calling? → Check settings, wiring, batteries
2. Indoor fan running? → Check blower motor, capacitor, control board
3. Outdoor unit running? → Check power, contactor, capacitor
4. Good airflow? → Check filter, coil, ductwork
5. Proper pressures? → Check refrigerant charge, restrictions
6. Compressor operating? → Check windings, overload, mechanical

Frequently Asked Questions