Vent, Valve & Volt
AC won't turn on, or the outdoor unit isn't running

You can fix this

Likely cause: a one-time breaker trip

The breaker tripped once and held after a reset. Keep an eye on it; if it trips again, call a technician.

Why this happens

Lennox and Trane both list a tripped breaker as a common reason an AC won't start. A single trip can follow a power surge or outage. Energy.gov suggests letting the unit cool for five minutes before resetting, which gives the system a moment to settle before it restarts.

How to fix it

  1. Leave the thermostat on Cool and confirm the outdoor fan is spinning.
  2. Check the air filter while you're at it. Trane lists a clogged filter as another cause of AC trouble, and recommends a new one every 1 to 3 months.
  3. Check the breaker again over the next day or two.
  4. If it trips a second time, leave it off and call an HVAC technician.

When to stop. A second trip means something in the system is drawing too much power. Don't keep resetting it.

If this does not fix it, tell the HVAC technician

  • Which breaker tripped and when.
  • Whether it happened after a storm or power outage.
  • Whether it has tripped again since.

Build the full list: what to tell the HVAC technician.

Start this troubleshooter over

References (every fact above comes from one of these pages)
  1. Lennox: Outside AC unit not turning on
  2. Trane: Air conditioner troubleshooting
  3. Energy.gov Energy Saver: Common air conditioner problems
  4. Trane: AC not blowing cold air

Vent, Valve & Volt is published by the team behind FourHome. Browse all FourHome replacement faucet and shower parts on the FourHome Amazon store.