AC Outdoor Unit Not Turning On: What to Check First

Short answer: If the indoor fan runs but the outdoor AC unit stays silent, check that the thermostat is set to Cool below room temperature, that the outdoor unit's breaker hasn't tripped, that the disconnect switch by the unit is on, and that a condensate float switch hasn't shut the system off. If all of that checks out, the usual causes are a failed capacitor, a bad contactor, a wiring or control problem, or a compressor or fan motor fault, which a technician needs to diagnose.

Safety note: the outdoor unit carries high voltage, and capacitors can hold a charge even with the power off. Don't open the access panel. If you smell burning or see scorching at the unit, turn the system off at the thermostat and schedule a visit. If you smell gas, leave the house and call 911 or Southwest Gas from outside.

First, what are you seeing?

The outdoor unit (the condenser) holds the compressor and the large outdoor fan. When the AC is cooling, both should run. Narrow it down:

• Nothing at all outside, indoor fan running: start with the checks below.
• Outdoor unit hums or buzzes, but the fan doesn't spin: often a capacitor or motor problem. See our AC capacitor guide.
• Outdoor unit clicks but won't start, or chatters: see our AC contactor guide.
• Outdoor unit runs but the air inside is warm: see AC blowing warm air.
• Nothing runs, inside or out: check the thermostat and the indoor unit's breaker first.

Safe checks you can do

Do these once, in order.

1. Thermostat

Set it to Cool and a few degrees below the room temperature. Make sure the screen is on. Many thermostats have a built-in delay of a few minutes before the outdoor unit starts, so give it five minutes. If the screen is blank or acting oddly, see our thermostat guide.

2. Breakers

The outdoor unit usually has its own two-pole breaker in the main panel, separate from the indoor unit. If it's tripped, reset it once. If it trips again, leave it off and call. Our AC tripping the breaker guide explains why.

3. The outdoor disconnect

Near the outdoor unit there's usually a small gray box on the wall. It holds a pull-out block, a lever or a breaker. Make sure it's in the on position. It sometimes gets switched off during yard work or a past service visit.

4. The condensate float switch

Many systems have a safety switch in the indoor drain pan or drain line. If water backs up, it cuts power to the outdoor unit while the indoor fan may keep running. Look for standing water in the pan by the indoor unit, or a drain line that isn't dripping outside while the AC runs.

5. The filter and the indoor coil

A badly clogged filter can let the indoor coil ice over. Some systems shut the outdoor unit down for protection. If you see ice on the refrigerant lines, turn the system to Off, leave the fan on On to thaw it, and see our frozen AC coil guide.

6. Look, don't touch

From outside the unit, check for obvious problems: a fan blade blocked by debris, chewed wiring, or a unit buried in plants. Keep a couple of feet of open space around it.

What usually causes it

If the checks above don't bring it back, a technician will usually test:

• The run capacitor. It helps the compressor and fan motor start and run. Heat is hard on capacitors, and failures are common in Las Vegas summers.
• The contactor. The relay that switches power to the outdoor unit when the thermostat calls for cooling. Pitted or burned contacts, or insects inside, can stop it from closing.
• Low-voltage wiring or a blown control fuse. The thin thermostat wire that runs to the outdoor unit can be damaged by yard work, pests or sun. A short can blow a small fuse on the indoor control board.
• Safety switches. Some units have high- or low-pressure switches that shut the unit off when something's wrong with the refrigerant system.
• The compressor or fan motor. Less common than the parts above, but possible, especially in older units.

Why it matters in Las Vegas heat

Outdoor units here work hard. NOAA's 1991–2020 normals put the average July high at Las Vegas's airport station at 104.5°F. Heat is hard on electrical parts like capacitors and contactors, and a home can warm up quickly when the AC stops. Don't keep cycling the thermostat to force it to start. Repeated hard starts on a failing part can damage the compressor.

What not to do

• Don't open the outdoor unit's panel or touch the capacitor.
• Don't keep resetting a breaker that trips again.
• Don't push the contactor in by hand.
• Don't hose down the unit's electrical box.

When to call

Call if the outdoor unit still won't run after these checks, if a breaker trips again, if the unit hums without the fan spinning, or if you see burn marks or smell burning. See our AC repair service or our Las Vegas AC repair page.

FAQ

Why is my AC fan running inside but not outside?

The outdoor unit isn't getting the signal or power to start. Common causes are a tripped breaker, a switched-off disconnect, a float switch tripped by a backed-up drain, a bad capacitor or contactor, or a wiring problem.

Where is the AC outdoor disconnect?

It's usually a small gray box mounted on the wall near the outdoor unit. Inside is a pull-out block, a lever or a breaker. Make sure it's in the on position.

Can a clogged drain stop the outdoor unit?

Yes. Many systems have a float switch that cuts power to the outdoor unit when water backs up in the drain pan or line, while the indoor fan may keep running.

Why does my outdoor AC unit hum but not run?

Humming with no fan or compressor start often points to a weak capacitor or a stuck motor. Turn the system off at the thermostat and call. Don't try to start the fan with a stick.

How long should I wait after resetting the thermostat?

Many thermostats delay the outdoor unit for a few minutes after a setting change to protect the compressor. Give it about five minutes before deciding it won't start.

What hours can I reach Colton?

Colton Air Conditioning is open Monday to Friday, 8am to 5pm. You can call or book online during those hours.

Colton Air Conditioning

Colton Air Conditioning, 6545 S. Fort Apache Rd Ste 135 #467, Las Vegas, NV 89148. Nevada contractor license 0092120. Hours: Monday to Friday, 8am to 5pm. Call (702) 738-6976 or book online during those hours.

Want routine maintenance on a schedule? See our Comfort Club maintenance plans: Gold, Platinum and Diamond tiers are all listed there.