Why Is My AC Not Working? Complete Troubleshooting Guide (2026)

An AC that "doesn't work" can mean one of three things: completely dead (no fan, no sound, nothing), blowing but not cold (covered in our why is my AC not cooling guide), or partially working (indoor fan runs but outdoor unit is silent — or vice versa). This guide covers the first and third — a dead AC and a partial AC.

Most "dead AC" calls in Rialto, Fontana, San Bernardino, Colton, and Rancho Cucamonga turn out to be ONE of five things. We've fixed all five hundreds of times. Walk through these in order — it'll take you 10 minutes max.

The 6-Step Homeowner Check (Do These First)

Step 1: Check the Thermostat

Stand at the thermostat. Confirm:

• Mode is set to COOL (not HEAT, not OFF, not FAN ONLY)
• Fan is set to AUTO (not ON — ON keeps the indoor fan running constantly without cooling)
• Set temperature is at least 5° below the current room temperature
• Screen is lit. If blank: battery-powered thermostats — replace the AA or AAA batteries. Hardwired smart thermostats (Nest, Ecobee, Honeywell) — a blank screen means the 24V power feed from the air handler has failed, which is a tech call.
• If smart thermostat: confirm it's not in "Away" mode, schedule override mode, or "Eco" hold.

Step 2: Check the Main Breaker Panel

Find your main breaker panel (garage, utility room, or outside the home). Look for breakers labeled AC, A/C, Air Conditioner, Air Handler, or Furnace. Most systems have TWO breakers: one for the outdoor condenser and one for the indoor air handler/furnace.

If any breaker is in the MIDDLE position (between ON and OFF), it's tripped. Push it fully to OFF first, then back to ON. If it trips again immediately — STOP. That's a short circuit somewhere (failed capacitor, burned compressor, damaged wiring) that needs a tech. Repeatedly flipping a tripping breaker can start a fire.

Step 3: Check the Outdoor Disconnect

Walk to the outdoor condenser unit. On the wall near it (usually within 6 feet) there's a gray metal box about the size of a tissue box — the disconnect. Open it. Inside is either a pull-out fuse holder or two fuses.

• If the pull-out is flipped/removed — push it back in correctly.
• If fuses look burned/blackened — they're blown. Don't replace without diagnosing why. Call a tech.
• If a landscaper or pool cleaner recently worked near the unit — they sometimes pull the disconnect "to be safe" and forget to put it back.

Step 4: Check the Condensate Float Switch

If your AC condensate drain clogs, the float switch trips and kills power to the entire system as a safety measure (to prevent water damage). Locate the indoor air handler — usually in a closet, attic, or garage. Look for a small float switch on the drain line near the indoor coil.

If the drain pan has standing water, your drain is clogged. Clear it: shop vac the outdoor end of the drain line for 60 seconds, or pour a cup of distilled vinegar into the drain access port. Reset the float switch (some have a manual reset button, others reset automatically once water drains).

Step 5: Check (and Replace) the Filter

A severely clogged filter doesn't usually kill the AC outright but it triggers safety lockouts on newer systems. If the filter is gray, dust-caked, or hasn't been changed in 6+ months, replace it. While you're at it, check that nothing is blocking return-air grilles (rugs, furniture, drapes).

Step 6: Listen at the Outdoor Unit

Set the thermostat to COOL and 5° below room temp. Within 60 seconds, you should hear the outdoor unit start: fan spinning, compressor humming. If you hear a humming or buzzing sound but nothing spins, the capacitor is dead — call a tech. If you hear NOTHING after a minute, the contactor or transformer failed — also a tech call.

If the 6-Step Check Doesn't Fix It

You've done what you safely can. What's left requires tools, training, and a license. Here's what a tech will check next, and roughly what each costs:

Failed Capacitor — $185-$285 installed

The #1 culprit in Inland Empire AC failures. Heat kills capacitors. A 5-year-old AC in Rialto fails capacitors as often as a 10-year-old AC in Long Beach. Symptoms: humming outdoor unit, no fan/compressor start, sometimes a faint electrical smell. Replacement takes 20 minutes once the tech arrives.

Failed Contactor — $185-$245 installed

The contactor is the relay that energizes the outdoor unit when the thermostat calls for cooling. It clicks audibly when it works. Burned/pitted contacts mean it doesn't make contact. Common in systems 8+ years old. Symptoms: silent outdoor unit, no humming, indoor fan works fine.

Blown Low-Voltage Fuse on Control Board — $185-$325 installed

The control board has a small (3-5 amp) fuse that protects against shorted thermostat wiring. When it blows, the entire system goes dead. Usually caused by a thermostat wire that got crushed during pest control, attic work, or rodent damage. Tech finds and fixes the underlying short, then replaces the fuse.

Burned Transformer — $285-$485 installed

The 24V transformer steps line voltage down to control voltage for the thermostat and relays. When it burns out, the thermostat goes dark and the system is completely dead. Often caused by the same shorts that blow the low-voltage fuse, just more severe.

Failed Compressor — $1,800-$3,200 or System Replacement

The compressor is the heart of your AC and the most expensive part. When it fails, it's often time to consider a full system replacement instead. Symptoms: humming or buzzing outdoor unit that immediately trips the breaker, oil stains around the outdoor unit, refrigerant smell, system 12+ years old. Get our AC installation quote before deciding.

What NOT to Do

• Don't open the outdoor unit. Capacitors hold 240V even when power is OFF. People die from this every year.
• Don't repeatedly flip the breaker. If it trips twice, there's a short. Stop trying.
• Don't add refrigerant yourself. Illegal in California without EPA certification. Also dangerous.
• Don't ignore burning electrical smells. Turn off the breaker at the panel and call us.
• Don't ignore breaker that won't stay on. That's a short — fire risk.

How Fast Should I Call?

Call immediately if any of these apply:

• Outside temperature is forecast above 95°F in the next 24 hours
• Anyone in the home has heart disease, COPD, or other heat-sensitive conditions
• Pets are inside (especially flat-faced breeds like bulldogs, pugs, persians)
• Infants or elderly residents are in the home
• There's a burning electrical smell from anywhere
• The breaker trips repeatedly
• Smoke or visible damage at the outdoor unit

BACS HVAC Services dispatches from our Rialto shop with same-day service across the Inland Empire. Our diagnostic visit is $89 (credited toward any repair you authorize). We're CSLB licensed #1122557, EPA Section 608 certified, and 100% HVAC-focused — we don't do plumbing, electrical, or general handyman. Book AC repair or call (909) 552-3189.