Quick answer
An AC that is not working after a power outage usually has a tripped breaker, a thermostat that rebooted with lost settings, or a 5-minute compressor lockout still counting down. If those check out, the restoration surge has likely blown the 3 A or 5 A fuse on the furnace control board or damaged the capacitor or contactor, both $150–$350 repairs.
If your AC is not working after a power outage, wait five minutes before touching anything. Most thermostats and many outdoor units hold the compressor off for about that long after power returns, so it does not restart against high pressure. A flashing snowflake or a wait message on the thermostat is that timer. If the system is still dead after ten minutes, check the thermostat, then the power, then the low-voltage fuse, in that order.
Thermostat rebooted, settings lost or compressor lockout running
The thermostat is the first place an outage shows. Smart and hard-wired thermostats reboot, and some come back in Off or Heat mode, with the schedule reset or still waiting to rejoin Wi-Fi. Set it to Cool, fan AUTO and a setpoint a few degrees below room temperature, then let the delay finish. A screen that stays dark is getting no 24-volt power, which points at the furnace breaker or the control-board fuse; a blank thermostat screen has its own checklist.
Check two breakers, the outdoor disconnect and the 3 A or 5 A control-board fuse
A central air conditioner depends on two circuits. The outdoor unit has its own double-pole breaker, and the furnace or air handler, which supplies the blower and the 24-volt control power, has another. Either can trip when power comes back.
- Find both breakers in the panel. A tripped breaker sits in the middle position; push it fully off, then on.
- Check the furnace power switch on or beside the cabinet. It looks like a light switch and gets bumped.
- Open the disconnect box on the wall beside the outdoor unit. Some have their own fuses or a breaker inside.
- Reset a breaker once only. If it trips again, leave it off: an AC that trips the breaker has a fault that repeated resets make worse.
When both breakers are on and the thermostat is still dead, the low-voltage fuse is the likely cause. Furnace control boards carry a small automotive-style blade fuse, violet for 3 amp or tan for 5 amp, that protects the 24-volt circuit. A surge or a momentary short as power returned can blow it, and with it gone nothing can call for cooling. Replace it only with the same rating; if the new one blows too, there is a short in the thermostat cable or the contactor coil that has to be found.
Surge and brownout damage: capacitor, contactor and control board
An outdoor unit that hums without starting, clicks, or runs with the fan stopped after an outage has a component that took the hit. The run capacitor is the usual casualty, followed by the contactor, whose contacts can burn or weld when the compressor tries to start on low voltage. Units with electronic boards, including inverter models, can lose the board.
- Capacitor replacement: $150–$350.
- Contactor replacement: $150–$350.
- Control board replacement: $400–$800.
- Diagnostic visit: $90–$150, credited to the repair if you proceed.
If the outdoor unit hums and the fan does not turn, switch cooling off at the thermostat. A stalled compressor overheats a little more each time it tries.
Brownouts and restoration surges on local hydro feeders
The outage itself is rarely what does the damage; the minutes around it are. During a brownout the voltage sags, and a running compressor draws more current, stalls and overheats. When a utility such as Burlington Hydro, Oakville Hydro or Alectra re-energizes a feeder, every fridge, sump pump and air conditioner on the street tries to start in the same instant, and the voltage dips and spikes while it settles.
The habit that prevents most of it costs nothing. When the lights flicker, dim or go out, set the thermostat to Off, and turn cooling back on ten minutes after the power has been steady.
When a condenser or whole-home surge protector is worth fitting
A surge protector is worth fitting when the equipment has expensive electronics or the supply is rough. That means inverter and variable-speed air conditioners and heat pumps, homes at the end of long rural lines in Caledon or Flamborough, and any house that has already lost a board or a capacitor to an outage. A condenser-mounted protector is wired at the outdoor disconnect; a whole-home unit goes at the electrical panel and covers the furnace board and everything else. Panel work belongs to a licensed electrician and falls under ESA rules.
Know the limit: a surge protector clamps voltage spikes and does nothing about the low voltage of a brownout. For a basic single-stage unit on a stable street, turning the thermostat off during outages gives most of the protection. Pricing depends on the panel and the unit, so ask for a written quote.
If the thermostat, both breakers and the fuse check out and the AC still will not run, it needs testing with a meter. A plain power-cycle is described in how to reset an air conditioner. ZK Mechanical does AC repair across Burlington, Oakville, Milton, Hamilton and Mississauga, and the emergency line is answered around the clock at (647) 801-1252; you can also book a visit online. Neighbouring faults are indexed under cooling answers.