What it is
A no-cool call is the most common ticket you'll run, and it's also where techs waste the most time guessing. The fix for that is a fixed order of operations. You don't start at the part you suspect — you start at the customer and walk the system in one direction until something doesn't do what it should. The first thing that fails to act is your suspect. Everything after it is innocent until you clear the thing in front of it.
This walkthrough gives you that order. Run it the same way every time and the call gets faster, not slower.
How it works
Cooling needs three things, and a no-cool is always at least one of them missing: a call (the thermostat asking for cool), power and controls (24V making the right things energize, line voltage feeding the loads), and the refrigeration side actually moving heat with adequate airflow over both coils. Your job is to figure out which of the three broke, and you do it by checking them in the order the system itself uses them.
The big mental split, once the unit is trying to run, is power problem vs. refrigerant problem. Those two branches have totally different tools and totally different fixes, and you want to commit to one branch before you start pulling panels.
In the field
Work it top to bottom:
- Confirm the complaint. Feel the supply air, check the thermostat setpoint vs. room temp, and ask what changed. "No cool" sometimes means "warm air from one register" or "it runs all day and won't catch up" — those aren't the same call. Set the stat to cool, setpoint well below room temp, fan on auto.
- Is anything running at all? Listen and look. Three buckets: nothing runs, indoor blower runs but outdoor doesn't, or everything runs but the air isn't cold. Each bucket sends you a different direction.
- Nothing runs → start at power and the call. Check the disconnect and breaker at the air handler/furnace and at the condenser. Check for 24V across R-C at the board. Check the float switch and any door switch. A tripped float from a clogged condensate drain is a classic "everything's dead" cause people overlook.
- Blower runs, outdoor unit dead → you've got a call but the condenser isn't getting it or can't act. Check for 24V on the Y circuit at the contactor coil. Got 24V and the contactor won't pull in? Suspect the contactor coil or a stuck/seized issue. No 24V at the coil? Walk it back — stat, Y wire, any safety in series (high-pressure switch, low-pressure switch), the transformer. Outdoor 24V present and contactor pulled in but compressor/fan still dead → now it's line voltage, capacitor, or the loads themselves.
- Everything runs, air isn't cold → now it's a refrigerant or airflow problem and the gauges come out. Before you condemn anything, rule out airflow — dirty filter, plugged evaporator coil, blower not moving air, dirty condenser. A starved or smothered coil mimics low charge. Then read suction and head pressure, superheat and subcool, and the temperature split across the indoor coil.
- Interpret the readings as a state, not a single number. Low suction + high superheat + low head usually means undercharge or a restriction. High head + high subcool means overcharge or a condenser that can't reject heat (dirty coil, dead fan, recirculating air). Normal-ish pressures with a weak split often means an airflow or capacity-vs-load issue, not a charge issue.
Normal values & targets
Ballpark on a healthy R-410A system on a moderate day, just so you know when a reading is "off":
- Supply-to-return temperature split: about 16–22°F at the air handler under normal humidity. A 6°F split screams airflow or low capacity; a 28°F split can mean low airflow over the coil.
- Suction pressure: roughly 115–135 psig (saturation ~38–45°F).
- Head pressure: roughly 350–425 psig, with condensing temp landing ~15–25°F over outdoor ambient.
- Control voltage: 24–28 VAC across R-C. Below ~21V and contactors/relays get unreliable.
Numbers move with weather, humidity, and load. Use them to spot what's abnormal, not as pass/fail gates.
Common faults & what they mean
- Float switch open / clogged drain: kills the whole call, looks like a dead system. Check it early.
- Bad run capacitor: outdoor fan or compressor hums and won't start, or starts and trips. Cheap, common, test it before condemning a motor or compressor.
- Failed contactor: chattering, pitted contacts, or won't pull in even with 24V on the coil.
- Dirty condenser / dead condenser fan: high head, high subcool, marginal or no cooling on hot days especially.
- Low charge / leak: low suction, high superheat, low-ish head, long run times, sometimes ice on the suction line.
- Restriction (TXV, filter drier, kinked liquid line): high superheat with low suction but the condenser side may look near-normal; a temperature drop across the drier is a tell.
Tech tips & gotchas
- Decide power branch vs. refrigerant branch before you pull the gauges out. Gauges tell you nothing about why a contactor isn't pulling in.
- Always clear airflow before you touch the charge. More "low on refrigerant" diagnoses are actually dirty filters and plugged coils than techs admit.
- If the indoor coil is iced, you can't read accurate pressures. Set fan to on, kill cooling, let it melt, fix the airflow or charge cause, then re-read.
- One reading is a data point; a set of readings is a diagnosis. Suction, head, superheat, subcool, and the air split together tell a story no single gauge can.
Safety / code notes
- Kill and verify power at the disconnect before touching line-voltage loads or capacitors; a "discharged" run cap can still bite — bleed it.
- Verify the equipment disconnect/whip is present and within sight of the unit per NEC §440 requirements.
- Wear eye protection on the gauges — R-410A liquid can flash-freeze skin and eyes.