AC Not Cooling? A Step-by-Step Diagnostic Checklist for Techs

A no-cool call goes fastest when you check things in the same order every time. Jumping straight to gauges skips the cheap, common causes and can lead you to add refrigerant to a system that actually has an airflow problem.

1. Confirm the call for cooling

• Thermostat set to Cool, setpoint below room temperature, and not in a delay or lockout period.
• Check for 24 V between Y and C at the air handler or furnace control board. If Y is dead at the board, the problem is the thermostat, the wiring, or a safety switch (like a float switch) breaking the circuit.
• On heat pumps, check that the thermostat's reversing valve setting (O or B) matches the equipment. A mismatch gives you heat when you call for cooling.

2. Confirm power at both units

• Breakers on, disconnects in, service switch on at the furnace or air handler.
• Check line voltage at the condenser contactor's line side. Low voltage (more than about 10% under nameplate) can keep a compressor from starting or cause it to trip.

3. Check indoor airflow before anything else

Low airflow causes low suction pressure, a frozen coil and poor cooling, and it looks a lot like a low charge on the gauges.

• Filter: dirty, wrong size, or an overly restrictive high-MERV filter.
• Blower: running at the right speed for cooling? A dirty blower wheel can cut airflow a lot.
• Evaporator coil: dirty or matted face, or ice. If the coil is frozen, shut the compressor off, run the fan to thaw it, and don't diagnose the charge until it's fully thawed. See frozen evaporator coil causes.
• Ductwork: closed dampers, crushed flex, disconnected runs. Measure total external static pressure if you suspect the ductwork.

4. Check the outdoor unit

With a call for cooling:

• Contactor pulled in? If not, check for 24 V at the coil. No 24 V points back to the control circuit or a safety (high- or low-pressure switch, defrost board lockout).
• Compressor running? Humming but not starting usually means a bad run capacitor, a locked compressor or low voltage. See AC compressor not starting.
• Condenser fan running? If the compressor runs but the fan doesn't, head pressure climbs fast. See condenser fan not spinning.
• Condenser coil clean? A dirty coil raises head pressure and cuts capacity.

5. Measure performance

Once air is moving and everything is running for 10–15 minutes:

• Temperature split across the indoor coil: on a typical system with normal humidity, return-to-supply drop is often around 15–22 °F. A low split suggests low capacity; a very high split suggests low airflow.
• Superheat and subcooling: compare against the manufacturer's target. Charge TXV systems by subcooling and fixed-orifice systems by superheat. Use the superheat & subcooling calculator and the target superheat calculator.
• Amp draw: compressor and fan motor amps against the nameplate RLA/FLA.

6. Read the readings together

Suction	Head	Superheat	Subcooling	Likely cause
Low	Low	High	Low	Undercharge (leak)
High	High	Low	High	Overcharge, or non-condensables
Low	Low/normal	High	High	Restriction (filter drier, metering device)
Low	Low	Low	Normal	Low indoor airflow
High	Low	Normal/low	Low	Inefficient compressor

These are patterns, not verdicts. Confirm with a second test before you replace parts or adjust the charge.

Safety: Lock out and tag out before touching electrical components, and discharge capacitors before handling them. Refrigerant work requires EPA 608 certification.

FAQ

Should I add refrigerant if the suction pressure is low?

Not until you've confirmed indoor airflow and checked superheat and subcooling. Low airflow and restrictions both cause low suction pressure. Adding refrigerant to those systems overcharges them.

What is a normal temperature split for an air conditioner?

On many residential systems at normal indoor humidity, the drop across the coil lands somewhere around 15–22 °F. High humidity lowers it, because more of the system's capacity goes to removing moisture. Use it as a quick check, not a charging method.

Why does the outdoor unit run but the house doesn't cool?

Common causes are a compressor that's tripped on its internal overload while the fan keeps running, low charge, a restriction, a dirty indoor coil, or low airflow. Check compressor amps first to confirm it's actually running.

Sources

• HVAC School: technical tips and diagnostics
• ACHR News: static pressure readings and system performance