Quick answer
To reset an air conditioner, set the thermostat to Off, switch the AC breaker off for 5 minutes, turn it back on, then set the thermostat to Cool. The compressor waits another 3 to 5 minutes before starting; that delay is built in. Reset once only. If the breaker trips again, there is a short or a failing compressor and the unit stays off.
To reset your air conditioner you cut its power, wait, and restore it in the right order. There is no hidden master button on most central systems. A reset clears a control board or thermostat that has hung after a power flicker, and it lets a compressor that tripped on overload try again. It repairs nothing, so the steps below end with what to do when it does not work.
The air conditioner reset, step by step
- Set the thermostat to Off. This removes the call for cooling so nothing tries to start as power returns.
- At the electrical panel, switch off the double-pole breaker labelled AC or condenser. Switch off the furnace or air handler breaker too, since the furnace board sends the cooling signal.
- Wait 5 minutes. Refrigerant pressures need that long to equalise, and a compressor forced to start against high pressure can stall.
- Turn both breakers back on, pushing each one firmly to On.
- Set the thermostat to Cool, a few degrees below room temperature.
- Wait again, hands off, for another 5 minutes.
Why nothing happens for the first 3 to 5 minutes
Most thermostats and many outdoor units have an anti-short-cycle delay that holds the compressor off for 3 to 5 minutes after power returns or after the last shutdown. Smart thermostats often show it on screen as a countdown or a waiting message. The indoor fan may start first. People assume the reset failed, flip the breaker again, and restart the timer each time.
After the delay you should hear the outdoor unit start, and the larger copper line at the unit should feel cold after ten minutes or so. If it does, the reset worked. Watch the next few cycles to be sure it holds.
Finding the outdoor disconnect and the high-pressure reset button
The outdoor disconnect is a weatherproof box, usually grey, on the house wall close to the condenser. Open the cover and you will find one of two types: a pull-out block with a handle, which you pull straight out to cut power, or a switch you flip. It does the same job as the panel breaker and is the one to use when you are standing at the unit. Put a pull-out back the way it came out, because many of them sit in the Off position when inserted upside down.
Some condensers also have a manual high-pressure reset: a small button, often red, on a pressure switch near the refrigerant lines. It pops when head pressure climbs too high, most often from a dirty coil or a stopped fan. If your unit has one and you can reach it without removing a panel, press it once after the unit has cooled for 30 minutes. Many residential units have no such button, and that is normal.
One reset only: when to stop and call
- The breaker trips again, at once or after a few minutes of running. That signals a short to ground, a seized compressor or a failed capacitor.
- The high-pressure button pops a second time.
- You smell burning or see scorch marks at the disconnect.
- The outdoor unit hums without starting.
- The unit runs but the coil ices or the air is not cold.
A breaker is not meant to clear the same fault over and over, and a compressor forced to restart against a mechanical fault pulls its locked-rotor current on every attempt. Repeated resetting is how a repairable fault becomes a burnt wire or a dead compressor, and it is a fire risk at the panel and the disconnect. The causes are sorted out in why an AC trips the breaker.
When a reset is the right move, and what comes next when it is not
- Right: after an outage or a flicker from your utility, whether that is Burlington Hydro, Oakville Hydro, Milton Hydro or Alectra, when the thermostat is lit but the system does not respond. See AC not working after a power outage.
- Right: after you have fixed an obvious cause, such as a plugged filter or a blocked outdoor coil, and the unit had tripped on overload.
- Wrong: as a weekly routine. A system that needs resetting more than once a season has a fault.
- Wrong: for a unit that is running but not cooling. Power cycling does not add refrigerant or thaw a coil.
A diagnostic visit is $90–$150 and comes off the repair bill when you proceed. The parts that most often sit behind a failed reset, the capacitor and the contactor, are $150–$350 each, and most AC repairs land between $150–$800.
If one reset did not bring it back, leave the breaker off and call ZK Mechanical at (647) 801-1252. Tell us what tripped and when, because it narrows the search. The AC repair page explains the visit, why an AC will not turn on covers the no-power cases, and you can request service online.