No-Cooling Repairs: Diagnosing a System That Will Not Cool

Running is not the same as cooling

The outdoor unit hums. The vents push air. The thermostat reads 84 and climbing. In Phoenix, that gap between "on" and "cooling" is where most July service calls live, and it is why we treat no-cooling as its own repair category rather than a symptom of everything. If you are standing in that house right now, start with the AC not cooling emergency page: it covers what to check and what to shut off before anyone arrives. This page is about what happens after the truck pulls up. Two broad versions of the complaint reach our dispatch. In the first, the system starts and runs but the air never gets cold. In the second, nothing outside starts at all, or it tries and gives up. The diagnostic paths overlap, but they begin in different places, and the technician needs to know which one you have before the first gauge comes out of the bag.

What we measure first

A no-cooling diagnosis is a sequence, not a guess. Ours runs in roughly this order, and each step rules something in or out:

• Thermostat call and low-voltage signal. Is the thermostat asking for cooling, and is that request reaching the outdoor unit? A weak or missing 24-volt signal ends the search before it starts.
• Supply voltage at the disconnect and contactor. Confirms power is present and the contactor is actually closing when asked.
• Capacitor microfarads under load. Compared against the rating on the can. A capacitor that reads well below its rating has usually been dying for a season.
• Compressor and fan amp draw. Read against nameplate. High amps on a locked compressor, zero amps on a dead fan motor, each tells a different story.
• Refrigerant pressures, superheat and subcooling. Not just "is it low" but why: charge, airflow, metering device or a restriction each print a distinct signature.
• Temperature split across the indoor coil. Return air minus supply air, with attic and duct losses accounted for.

Together those readings point to one component, or to a chain of them. We do not replace parts on a hunch, and we do not add refrigerant to a system we have not measured.

Common causes, ranked

Ranked by how often we find them on Phoenix no-cooling calls, most frequent first. Heat drives this list. A 155-degree attic and a condenser sitting in the sun on the west side of a block-and-stucco house age components faster than any manual assumes.

1. Failed run capacitor

The single most common outdoor failure in the Valley. The outdoor fan may spin slowly or not at all, the compressor may hum and trip on overload, or the whole unit may cycle in short bursts. Confirmed with a meter, replaced with a matched part.

2. Pitted or welded contactor

The contactor is the switch that sends power to the compressor and fan. Its contacts arc every cycle, and dust from a haboob does not help. A pitted contactor delivers unreliable power; a welded one will not let the unit shut off.

3. Low refrigerant charge

Almost always a leak, not evaporation. The system cools poorly, then not at all, and often ices over along the way. A proper fix means finding the leak, which is its own discipline covered in our refrigerant leak repair guide.

4. Frozen evaporator coil

A block of ice on the indoor coil stops airflow entirely. The cause is upstream: a clogged filter, a failed blower, low charge or a closed damper. Thawing it is step one; finding what froze it is the repair.

5. Failed condenser fan motor

Without the outdoor fan, the compressor cannot reject heat and shuts itself down on high pressure. Common on units that face afternoon sun and run fourteen hours a day.

6. Control board or relay failure

Less common, more expensive to chase. A board that drops the cooling call intermittently is the kind of fault that hides from a single visit and shows up on the second.

When this repair is worth it

A capacitor, contactor or fan motor on a system under twelve years old is almost always worth fixing. The part is modest, the labor is short, and the system returns to its prior state. The math changes when the failed component is the compressor, or when a refrigerant leak sits inside an evaporator coil on an R-22 system. At that point you are choosing between a large repair on old equipment and a replacement that resets the clock. Our repair-vs-replace guide walks through that decision without a thumb on the scale, and the estimates and financing page explains how the replacement side gets quoted. Two honest notes. First, if your system cooled fine until a filter clogged, the repair is a filter, and we will say so. Second, most no-cooling calls in June trace back to something a spring tune-up would have caught. A maintenance visit measures the same capacitor and contactor we measure on an emergency call, on a day when nobody is sweating.

What this page cannot do

It cannot tell you which of the six causes above is yours. Two systems with identical symptoms can have opposite faults, and a web page cannot read microfarads or pressures. An on-site evaluation can, and it ends with a flat-rate quote for the specific repair before any work begins. Whether your equipment is a conventional split system or a package unit on the roof, the diagnostic sequence is the same. Only the ladder changes.