AC Not Cooling: A Diagnostic Flowchart
"It runs but it won't cool" is the call you get all summer. This is the structured walk — thermostat, power, airflow, then gauges — that finds the fault fast instead of throwing parts at it.
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In This Guide
Why Order Matters
The fastest way to waste an hour on a no-cooling call is to lead with the gauges. Manifold readings only mean something once you know the system is being told to run, has power, and is moving air across both coils. Skip those and a low suction pressure will fool you into adding refrigerant to a system that is really just choked by a plugged filter.
So we work outside-in and cheapest-first: thermostat, then power and controls, then airflow, then refrigerant, and only then the compressor. Each step either clears a whole category of faults or hands you the next clue. That is what turns a scattershot "it's not cooling" into a decision tree you can finish in fifteen minutes.
Field rule: The single most common cause of a no-cooling call is restricted airflow — usually a filter no one changed. The second is a failed run capacitor. Neither is a refrigerant problem, and both live upstream of your gauges in this flow.
Step 1: The Thermostat Is Actually Calling
Before you touch the outdoor unit, confirm the equipment is being told to cool. Plenty of "broken AC" calls end here.
- Thermostat set to COOL and setpoint at least a couple degrees below room temp.
- Dead or dying batteries on a battery-powered stat — a classic no-call, no-cool.
- At the air handler board, confirm 24V between Y and C when there's a call. No 24V means the problem is the thermostat, wiring, or the transformer — not the condenser.
- Check that a tripped float switch or high-pressure lockout hasn't opened the Y circuit.
If Y is energized and the outdoor unit still sits dead, you've just proven the fault is downstream in power or the compressor circuit. Move on.
Step 2: Power and Controls
Now you're at the condenser. The question is simple: is the compressor and condenser fan getting the voltage they need, and are they drawing current?
- Disconnect and breaker: confirm line voltage (typically 208-240V) at the disconnect and that the breaker isn't tripped or bad.
- Contactor: with a call for cooling, verify 24V on the coil and that the contacts pull in. Pitted or welded contacts are a very common failure — welded contacts keep the compressor running even with no call.
- Run capacitor: discharge it, then measure microfarads. Replace if the reading is more than 6% off the rated MFD. A weak dual cap is a top-three cause of a compressor that hums and trips on overload but won't start.
Capacitor quick reference
Match the nameplate MFD; you may go to a higher voltage rating but never lower. A 440V cap can replace a 370V cap, not the reverse. Common dual-run sizes: 3-ton around 40/7.5 MFD, 5-ton around 60/5 MFD.
If the compressor hums for a second then trips, suspect the cap or a locked rotor. If it's dead silent with no voltage across the contactor load side, work back through the contactor, low-voltage fuse, and safeties.
Step 3: Airflow Before Refrigerant
The unit runs but the air is warm. Do not put gauges on yet — verify airflow first, because low airflow mimics an undercharge and can freeze the coil solid.
- Filter: pull it. A collapsed or gray-matted filter is the number-one no-cooling cause. Replace it before doing anything else.
- Evaporator coil: look for ice or a dirty coil face. A frozen coil means the system has been running starved of air or refrigerant — thaw it fully before charging.
- Blower: confirm the indoor blower runs and moves air; a failed capacitor or seized bearing kills airflow.
- Registers/dampers: closed supplies and crushed flex duct starve the coil.
Two measurements tell you whether airflow is right:
Temperature Split (Delta-T)
Target 14-22°F in cooling. Above 22°F suggests low airflow (or low charge); below 14°F suggests too much airflow or a capacity problem.
Total External Static
Measure across supply and return plenums. Most residential systems should be at or under 0.50 iwc; anything past 0.80 iwc means a real restriction.
Watch for the frozen-coil trap
A coil iced from low airflow gives you low suction pressure and high superheat — the exact signature of an undercharge. Add refrigerant here and you'll overcharge the system the moment airflow is restored. Always fix airflow and thaw the coil before judging charge.
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Step 4: Gauges, Superheat, and Subcooling
Airflow is confirmed and the coil is clear. Now the manifold earns its keep. Note suction and head pressure, then convert both to saturation temperatures on the PT chart for the refrigerant in the system. On R-410A, roughly 118 psig suction reads about 40°F evaporator saturation, and about 400 psig discharge reads near 110°F condenser saturation on a warm day. R-454B runs within a few percent of those numbers.
The two numbers that decide the call
Target 8-14°F on a TXV system.
Target 10-18°F (confirm the manufacturer's spec).
On a TXV system, subcooling is your charge indicator and superheat should hold steady in range. On a fixed-orifice system, superheat is the charging target and you use the manufacturer's charging chart (or the target-superheat method: 3 × WB + (80 − OD DB) − 1). Read both numbers before you decide anything — one alone can lie.
Reading the Pressure Patterns
This is where the diagnostic pays off. The combination of suction, head, superheat, and subcooling points to the fault. Here are the patterns you'll see most on a no-cooling call:
| Suction | Head | Superheat | Subcool | Likely Fault |
|---|---|---|---|---|
| Low | Low | High | Low | Undercharge / leak |
| Low | Low | High | Normal/High | Low airflow (evap) or restriction |
| Low | High | High | High | Liquid-line restriction / plugged TXV |
| High | High | Low | High | Overcharge |
| Normal/High | High | Normal | High | Dirty condenser / non-condensables |
| High | Low | High | Low | Weak / valved-out compressor |
| High | Low | Low | Low | TXV stuck open / floodback |
A helpful sanity check on the condenser: the condenser split (saturation temp minus outdoor ambient) should sit around 15-25°F. A split above 30°F with high head points straight at a dirty condenser coil or an overcharge — spring debris after the winter is the usual suspect.
Step 5: The Compressor
You only land here after airflow, charge, and metering are ruled out. The classic weak-compressor signature is high suction, low head, low subcooling — the pump can't build the pressure differential because internal valves or scrolls are worn or bypassing.
- Compression ratio: divide absolute discharge by absolute suction (add 14.7 psi to each gauge reading). Normal is roughly 2.5:1 to 3.5:1. A ratio well below that on a warm day suggests valve or scroll inefficiency.
- Amp draw: compare running amps to nameplate RLA. Drawing above RLA, or under about 50% of it, both flag trouble.
- Locked rotor: hums, trips overload, high locked-rotor amps — confirm the cap and start components first before condemning the compressor.
Safety: You need an EPA Section 608 certification to recover, evacuate, and charge. If the system uses an A2L refrigerant like R-454B, follow A2L handling practices — no open flames near a potential leak, verify leak detection and ventilation, and use A2L-rated recovery equipment.
Worked Example: The Warm-Air Call
The call:
3-ton R-410A split, outdoor unit running, indoor blower running, but the supply air is only mildly cool on a 92°F afternoon.
Step 1-2: Thermostat calling, 24V at Y confirmed. Contactor pulled in, capacitor reads 39.5 MFD against a 40 MFD rating — within 6%, good.
Step 3: Filter is clean, coil is clear, no ice. Delta-T measures only 9°F (target 14-22°F). Static is normal at 0.45 iwc, so airflow is adequate — the low split isn't an air problem.
Step 4: Gauges read 105 psig suction (≈36°F sat) and 340 psig head (≈100°F sat). Suction line temp 55°F, liquid line temp 92°F.
Superheat = 55 − 36 = 19°F (high)
Subcooling = 100 − 92 = 8°F (low)
Step 5: Pattern = low suction, low-ish head, high superheat, low subcooling. That is the undercharge/leak column in the table.
Diagnosis: Low charge — find the leak
High superheat with low subcooling and a weak delta-T is a starved evaporator with too little refrigerant. Because airflow already checked out, this isn't a false undercharge. Leak-check, repair, evacuate, and weigh in the nameplate charge — don't just top it off and leave.
Notice how each step eliminated a whole branch: the thermostat and power cleared "no call" and electrical, airflow cleared the frozen-coil trap, and only then did the gauges give a reading you could trust. That's the whole point of working the flow in order.
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