The outdoor unit (condenser) needs three things to start: line voltage at the disconnect, a 24-volt thermostat call from indoors, and a working contactor closing the circuit to the compressor and fan. A bad capacitor can let the unit hum without spinning. A blown low-voltage transformer or open contactor coil keeps it silent. Walk through the checks below before assuming the compressor itself failed — compressors rarely fail without first showing other symptoms.

Quick checks

  • Thermostat is set to Cool, fan on Auto, setpoint at least 3°F below room temperature.
  • Indoor blower is running but the outdoor unit is silent.
  • You hear clicking at the outdoor unit but nothing starts.
  • You hear humming at the outdoor unit but the fan doesn't spin.
  • Breaker at the main panel labeled AC or condenser is tripped.
  • The pull or breaker in the gray disconnect box next to the outdoor unit is in the off position.
  • Bulging or oozing capacitor visible inside the unit (don't touch — call a tech).

Step by step

  1. Set the thermostat to Cool. Drop the setpoint 5°F below the room temperature. The indoor blower should start within a minute and the outdoor unit within a few minutes.
  2. Replace the thermostat batteries if it runs on batteries. A weak battery can lose the cooling signal without showing a clear error.
  3. Walk to the main electrical panel. Find the breaker labeled AC, A/C, or condenser. If it's mid-position, flip it fully off, wait 30 seconds, then back on. Don't reset a breaker more than once if it trips again — a repeating trip is a real fault.
  4. Walk to the outdoor unit. The gray disconnect box is mounted within sight of the unit. Open it. Confirm the pull, switch, or breaker inside is in the on position.
  5. Check the inside of the unit by looking through the top grille without removing anything. If the fan blade is visible and the unit is silent with the thermostat calling, the contactor or low-voltage circuit isn't engaging.
  6. If you hear a low hum but the fan blade isn't spinning, shut the system off at the thermostat immediately. This is almost always a failed run capacitor and continuing to run damages the compressor.
  7. If the unit is silent and the breaker is on, and the indoor unit is calling for cool, it's a 24-volt signal problem (thermostat wire, contactor coil, or transformer) or a high-voltage problem (contactor, fuses at the disconnect).
  8. Call a licensed HVAC technician with the symptoms documented.

Why DIY stops at the disconnect cover

Capacitors store high-voltage charge even with power off — touching the terminals on a charged capacitor can cause serious injury. Contactors are 240-volt components with exposed line terminals. Refrigerant lines hold high pressure and any leak triggers EPA-regulated handling requirements. Trane's published guidance specifically says capacitors and the electrical circuit inside the unit should be tested by a trained HVAC professional. Cover everything that needs the cover off to a tech.

Symptom branches

  • Indoor blower runs, outdoor unit is silent. No 24V signal reaching the contactor, blown low-voltage fuse on the air handler board, or open contactor coil. Service.
  • Unit hums but the fan doesn't spin. Bad run capacitor or seized fan motor. Don't keep running it.
  • Unit clicks every few seconds. Capacitor failing or contactor chattering from low voltage.
  • Unit started this morning, now nothing. Tripped breaker, hot weather pushed components into thermal shutdown, or contactor stuck open.
  • Breaker trips when the unit tries to start. Stop trying. Internal short, locked compressor, or bad wiring. Service.
  • Fan spins but compressor doesn't. Compressor capacitor, compressor overload, or compressor failure. Service.
  • Unit runs constantly without cycling off. Thermostat issue, low refrigerant, or stuck contactor. Service if it persists.

What to check before you call

  • Note the exact model number printed on the data plate on the side of the outdoor unit (starts with letters like 2TT, 4TTR, etc.).
  • Note the symptom: silent, humming, clicking, fan spinning but no cooling.
  • Note how long the system has been installed.
  • Note any recent events — thunderstorm, power outage, breaker work, thermostat change.
  • Replace the air filter at the furnace or air handler so the tech doesn't have to flag it on top of the real issue.
  • Clear leaves and debris from around the outdoor unit so the tech can work safely.

When to call for service

  • Any humming with no fan rotation.
  • Any clicking with no startup.
  • Breaker that trips on restart.
  • Bulging, leaking, or burned-looking capacitor visible through the grille.
  • Burning smell at the outdoor unit.
  • Hissing or oily film at line connections — possible refrigerant leak.
  • Trane residential warranties vary by model and registration (10-year parts is common with registration). Find a Trane dealer at trane.com or call Trane customer service at 1-800-945-5884.

Good maintenance rhythm

  • Monthly during cooling season: replace the air filter at the indoor unit; clear plant growth and debris around the outdoor unit.
  • Yearly: professional AC tune-up in spring — capacitor test, contactor inspection, coil cleaning, refrigerant pressure check.
  • Yearly: rinse the outdoor condenser coil with a hose at low pressure from outside in.
  • Every cooling season: replace thermostat batteries.
  • Every 10 to 15 years: budget for replacement; older capacitors fail predictably with age and heat exposure.
  • Ongoing: keep 2 feet of clearance around the outdoor unit at all times.
  • After a major storm: visual check for damage before restarting the system.
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