The indoor air handler is running — the blower is on, air is moving through the house — but nothing is happening at the outdoor unit. The compressor is silent, the condenser fan isn’t turning.
Before calling a tech, work through these six checks. Three of them are free. One is a $25 part. Only the last two need a service call.
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Check 1: Wait 5 minutes (free)
If the AC just shut off and you’re trying to restart it: wait.
Modern thermostats and control boards have a compressor protection delay — a 3–5 minute lockout that prevents the compressor from restarting immediately after shutoff. This protects the compressor from restarting under pressure differential before the refrigerant pressures equalize.
Set the thermostat to cool, wait 5 full minutes, and watch the outdoor unit. If nothing happens after 5 minutes: continue.
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Check 2: Breaker and disconnect (free)
The outdoor unit has two separate power sources to check.
Main panel breaker: Find the breaker labeled “AC,” “condenser,” or “heat pump” in your electrical panel. It should be fully in the ON position. A tripped breaker sits in the middle or clicks to OFF when you touch it. Reset it: push fully to OFF, then fully to ON.
Outdoor disconnect: There’s a grey metal box mounted on the wall near the outdoor unit — the disconnect. Open the cover and check:
- If it has a pull-out block: confirm the block is fully seated with the power contacts connected.
- If it has a switch: it should be in the ON position.
- If the disconnect fuses are blown: replace with the correct amperage (printed on the disconnect) — fuse holders pull out of the disconnect block.
If the breaker trips again immediately after reset: a serious electrical fault (compressor short, ground fault) — do not reset again. Call a tech.
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Check 3: The thermostat call (free)
Confirm the thermostat is actually sending a cooling call:
- Is the system set to COOL, not HEAT, FAN ONLY, or OFF?
- Is the setpoint below current room temperature?
- Is the thermostat display active (not blank from dead batteries)?
- Set the setpoint 5°F below current room temp to force a definitive cooling call.
Also confirm the indoor unit is running. If the indoor air handler isn’t running either, the problem is power or thermostat — not the outdoor unit specifically.
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Check 4: The condenser fan — is it the fan or the compressor? (free)
Stand near the outdoor unit and listen carefully:
- No sound at all (no hum, no click): the contactor isn’t closing. Skip to Check 5.
- Hum but no fan or compressor starting: the contactor is closed (you hear the attempt) but components aren’t starting. Likely a failed capacitor.
- Fan spinning, compressor humming but not starting: run capacitor has failed — compressor can’t get enough starting current.
- Fan not spinning, compressor running: fan capacitor or fan motor has failed.
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Check 5: The capacitor (DIY-accessible, $15–40 part)
The run capacitor is the most common reason an outdoor unit won’t start. It provides the phase-shifted current motors need to start and run. When it fails:
- The compressor hums but won’t start
- The condenser fan doesn’t spin (or spins slowly)
- The unit may trip the breaker on startup
To check: with power off (disconnect pulled and breaker off), open the outdoor unit access panel. The capacitor is a silver or blue cylinder, typically 2–6 inches tall. Look for a visibly swollen or bulging top — a flat top is correct; a rounded top means the capacitor has failed internally.
For testing with a multimeter and replacement procedure: see HVAC Capacitor: Symptoms, Testing, and Replacement.
Safety note: discharge the capacitor before touching any terminals. Capacitors hold 300–400V DC even with power off. See the capacitor guide for the discharge procedure.
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Check 6: The contactor (DIY-accessible, $15–35 part)
The contactor is a high-amperage relay — the switch that connects line voltage to the compressor and fan when the thermostat calls. If the contactor’s contacts are pitted, burned, or stuck open, no voltage reaches the outdoor components.
Signs of a contactor problem:
- No hum at the outdoor unit (no current reaching it at all)
- Contactor visible through the access panel with burned or pitted contact faces
- Outdoor unit won’t start despite good capacitor
With power off, open the outdoor unit access panel. Find the contactor — a rectangular device with large wires connected to top (line) and bottom (load) terminals. Inspect the contact faces through the small opening in the side. Pitting, burning, or visible damage = replace.
For testing and replacement procedure: see AC Contactor: Symptoms and Replacement.
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If checks 1–6 don’t resolve it: call a tech
The remaining causes require professional equipment to diagnose:
- Compressor failure: the compressor itself has seized or failed internally. A tech confirms with current measurements and refrigerant pressures. Compressor replacement or system replacement.
- Low refrigerant pressure tripping the low-pressure safety: refrigerant undercharge from a leak causes low suction pressure; the low-pressure switch cuts the compressor. Requires gauges, leak detection, and EPA 608.
- High-pressure fault: dirty condenser, refrigerant overcharge, or failed fan can cause high-pressure cutout. Tech diagnoses with gauges.
- Control board failure: the board that sends signals to the outdoor unit. Requires diagnosis and replacement.
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Quick reference
| Observation | Most likely cause | DIY? |
|---|---|---|
| Nothing at all — no hum, no click | Breaker, disconnect, or contactor | Check 2, then Check 6 |
| Compressor hums but won’t start; fan not running | Run capacitor failed | Check 5 |
| Fan not spinning, compressor running | Fan capacitor or fan motor | Check 5 (fan side) |
| Outdoor unit runs briefly then shuts off | High-pressure trip (dirty coil/hot day), refrigerant | Clean condenser; call tech |
| Breaker trips immediately | Short circuit — compressor or wiring | Call tech |
| Outdoor unit fine, just no cooling | Indoor unit / refrigerant issue | AC not cooling guide |
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FAQ
The outdoor unit fan is spinning slowly. Is that bad? Yes — a slow condenser fan indicates a failing fan capacitor or fan motor. The condenser can’t reject heat with a slow fan, leading to high-pressure faults and compressor damage. Replace the capacitor first (it’s cheap and often the cause of slow fan); if fan speed doesn’t improve with a good capacitor, the fan motor is failing.
My outdoor unit is making a loud hum when it tries to start. What is that? The compressor is trying to start but can’t get enough starting torque — classic bad capacitor. The capacitor isn’t providing the phase-shift current needed. The unit will typically trip the breaker or the compressor’s thermal overload after 20–30 seconds if it can’t start. Replace the capacitor first.
Can my outdoor unit run without the indoor unit? No — the refrigerant circuit connects indoor and outdoor units. Running the outdoor unit without airflow across the indoor coil (no indoor unit running) would cause extremely low suction pressure and freeze or starve the compressor. Modern systems have safety controls that prevent this, but the point is moot: if both aren’t running together, cooling isn’t happening.
Should I cover my outdoor unit in winter? Generally no. Covering the outdoor unit in winter can trap moisture and pests inside. If you have a heat pump, never cover it — it operates in winter for heating. If you have a cooling-only AC, covering is unnecessary; modern units are designed for outdoor exposure year-round.
