Window AC Repair: Why It Won’t Cool, Won’t Turn On, or Won’t Stop Leaking

Your window air conditioner isn’t doing its job. Maybe it runs but blows warm air. Maybe it won’t turn on at all. Maybe it’s leaking water onto your floor, or making a noise that’s driving you mad. It’s the middle of summer, the temperature is climbing, and the machine that’s supposed to keep one room bearable has stopped cooperating.

Window air conditioners are remarkably simple machines. They contain the same components as a refrigerator — a compressor, a condenser coil, an evaporator coil, and a fan — just arranged differently and sized for a single room. That simplicity means most faults are diagnosable without specialist equipment, and many are fixable for well under $50.

The most common cause of poor cooling is simply a dirty filter or clogged coils. Restricted airflow reduces heat exchange and the machine can’t cool effectively. It takes 30 minutes to fix and costs nothing. If the machine hums but won’t start, the compressor capacitor is the usual culprit — a $15 to $30 part. If it leaks water, the unit is probably tilted the wrong way or the drain path is blocked. And if the compressor has genuinely failed, the repair usually exceeds the value of the unit, and replacement is the smarter call.

Common Symptoms

AC runs but blows warm or room-temperature air
AC won’t turn on at all — no fan, no compressor, no display
AC hums but the compressor doesn’t start
AC cools weakly — the room never gets comfortable
Water leaking inside the room or pooling excessively outside
AC short cycles — turning on and off every few minutes
Ice forming on the evaporator coil
Loud rattling, buzzing, or grinding noise
AC trips the circuit breaker when it starts

What Causes a Window AC to Stop Working

Dirty filter or clogged coils — restricted airflow reduces cooling efficiency. The most common cause of poor cooling by a wide margin.
Failed compressor capacitor — the compressor hums but won’t start. A $15 to $30 part.
Failed fan motor — the blower or condenser fan has seized or burned out.
Refrigerant leak — the sealed system has lost its charge. Not a DIY repair.
Failed compressor — the compressor has failed internally. Usually a bin decision.
Blocked drain path — condensate can’t escape, causing water to pool and leak inside.
Incorrect tilt — the unit must slope slightly downward toward the outside.
Failed thermostat or control board — the AC doesn’t cycle correctly.
Failed thermal overload — the compressor trips repeatedly, causing short cycling.

What the Guide Covers

The Window AC Repair guide is a complete 15-page diagnostic reference covering every common fault:

Filter and coil cleaning — the free fix that restores cooling in most cases
Compressor capacitor testing — with the correct discharge procedure and multimeter tests
Fan motor diagnosis — testing windings, checking blade condition, and identifying a seized motor
Water leak diagnosis — AC tilt correction, drain hole clearing, and slinger ring inspection
Noise diagnosis — tightening hardware, checking fan blades, and inspecting compressor mounts
Short cycling diagnosis — thermostat testing, condenser cleaning, and overload protector checks
Freezing up diagnosis — airflow vs refrigerant causes, and how to tell the difference
Power supply and control board checks — cord, switch, thermostat, and board faults
Refrigerant leak identification — recognising the symptoms and understanding the repair reality
Repair-versus-replace framework with cost thresholds for every fault

What You’ll Learn

You’ll learn the diagnostic order that saves time — start with the filter and coils because they account for most cooling complaints, then test the capacitor, then work into the fan and compressor. You’ll learn how to test a compressor capacitor safely, how to measure fan motor windings, and how to tell a refrigerant fault from an airflow fault.

You’ll learn the correct AC tilt that prevents water leaks, how to clear a blocked drain path, and why the slinger ring matters. You’ll learn how to diagnose a short cycling AC and how to tell the difference between an oversized unit and a genuine fault.

And you’ll learn the two thresholds that matter most: when a compressor or sealed system fault means replacement rather than repair, and how to make that call rationally.

Repair or Bin?

A dirty filter costs nothing to clean. A compressor capacitor costs $15 to $30. A fan motor costs $40 to $80. A control board costs $50 to $100. Against a window AC worth $150 to $500, most of these are worth doing.

The bin decision comes at the compressor and sealed system. A refrigerant leak or failed compressor typically costs $300 to $500 to repair professionally — which on a basic countertop window unit is more than the machine is worth. The guide walks through both scenarios with worked examples.

Frequently Asked Questions

Why is my window AC running but not cooling?

The most common cause is a dirty filter or clogged condenser coils. Restricted airflow prevents proper heat exchange. Clean both, and cooling is usually restored. If that doesn’t fix it, the compressor capacitor is the next suspect.

Why does my window AC hum but not start?

A failed compressor capacitor. The compressor is receiving power but can’t start spinning. The capacitor costs $15 to $30 and takes about 30 minutes to replace.

Why is my window AC leaking water inside?

Usually the unit is tilted the wrong way. A window AC should slope slightly downward toward the outside so condensate drains out. If it’s level or tilted inward, water pools and leaks into the room.

Why does my window AC keep turning on and off?

Short cycling is usually caused by a dirty condenser coil causing the compressor to overheat, or by a faulty thermostat. If the unit is oversized for the room, that’s a replacement issue rather than a repair.

Is it worth repairing a window AC?

For filter, capacitor, fan, and control board faults, yes. For compressor or refrigerant faults, usually not on a basic unit — the repair cost exceeds the replacement cost.

Get the Complete Window AC Repair Guide

Stop sweating. This guide diagnoses every common window AC fault, tells you what to test, and helps you decide whether to repair or replace.

Window AC Repair — $7.95

Know the fault. Fix it properly. Or know when it’s time for a new one.

Related Guides

Portable AC Repair — $7.95
The companion guide for portable units. Covers the same cooling system faults plus the condensate management and exhaust hose issues unique to portable ACs.

Dehumidifier Not Collecting Water — $7.95
Same category, similar sealed system and airflow diagnostics. Covers the most common causes including temperature range, filter blockage, and compressor faults.

Fridge Leaking Water Inside — $7.95
Same condensate and drainage principles applied to a refrigerator. Covers defrost drain blockages and drain pan faults.

Refrigerator Not Cooling But Freezer Working — $7.95
Same shared-cooling-system diagnostic logic. Useful if you’re building confidence in sealed system and airflow diagnosis.