Central A/C & heat pumps

AC compressor not starting

A compressor that won't start can be caused by several components around it; the compressor itself failing is usually the last thing to suspect, not the first.

Difficulty
Advanced
Time
Diagnosis and repair by pro
DIY cost
$0
Pro cost
$200–$1200 (higher if compressor itself has failed)

Symptoms

  • Outdoor fan may run but no compressor sound (a deeper hum or vibration)
  • AC blows air but it isn't cold
  • System may click and try repeatedly without starting

Likely causes

  • CommonFailed start or run capacitorThe compressor needs a capacitor's extra boost to start; without it, it can't turn on at all.
  • CommonFailed contactor not sending power to the compressorThis relay switches power to the outdoor unit and can fail with worn or pitted contacts.
  • PossibleCompressor overload protector trippedA built-in safety can shut the compressor down if it's overheating or drawing too much current.
  • Less commonSeized or failed compressorAfter ruling out capacitor and contactor, the compressor itself may have failed internally.

What to do

  1. Check that the outdoor fan is running; if the fan runs but there's no cooling and no compressor sound, focus on the compressor circuit.
  2. Turn off power at the disconnect before any closer inspection.
  3. Note if the system attempts to start repeatedly or clicks without engaging, which points to the contactor or capacitor.
  4. Don't attempt to test or bypass the contactor or capacitor yourself; these carry line voltage and stored charge.
  5. Call an HVAC technician to test the capacitor, contactor, and compressor windings in sequence to find the actual cause.

Tools and parts

AC contactorSearch: “air conditioner condenser contactor replacement”
AC dual run capacitorSearch: “air conditioner dual run capacitor correct rating”

When to call a pro

Call an HVAC technician to diagnose the compressor circuit; a failed compressor itself can be a significant repair or point toward system replacement.

Safety

Use caution

Never test compressor circuits yourself; they carry line voltage and can include stored capacitor charge.

Related guides

Operator provides AI-generated troubleshooting information. It can be wrong. For safety-critical, electrical, gas, structural, medical, automotive safety, or other high-risk problems, consult a qualified professional.