Quick answer
When an AC will not turn on, the cause is usually electrical and usually cheap: a thermostat not calling for cooling, a tripped breaker or blown fuse in the outdoor disconnect, a failed capacitor or contactor, or a safety switch on the condensate drain. Check the thermostat and breakers first. If the outdoor unit hums but never starts, stop and call a technician.
If you're asking why your AC is not turning on, start by noticing exactly what is and isn't happening. There are three different problems hiding behind that one complaint: nothing runs at all, the indoor fan runs but the outdoor unit is silent, or the outdoor unit hums or clicks but the fan never spins. Each points at a different part, and the first two are often something you can sort out yourself in 10 minutes.
Nothing runs at all: check the thermostat and the furnace power
If the indoor blower is silent and the outdoor unit is dead, the cooling call is not reaching the equipment. Confirm the thermostat is set to Cool with the setpoint at least 2°C below room temperature, and swap the batteries if it is a battery model; a dim or blank screen is the giveaway. Many thermostats also hold a five-minute compressor delay after a power blip, so wait it out before deciding anything is broken.
- The furnace power switch: the indoor half of your AC is the furnace blower. That light-switch beside the furnace gets knocked off by a laundry basket more often than you would think.
- The furnace breaker: usually a 15-amp single breaker in the panel, separate from the AC's own breaker.
- The 3-amp blade fuse on the furnace control board: it protects the 24-volt circuit and blows when a thermostat wire shorts. If a new fuse blows immediately, a low-voltage wire is shorted somewhere and needs tracing.
- The condensate float switch: on newer installs a small switch in the drain pan or line cuts the cooling call when water backs up. It should never be bypassed; find out why an AC leaks water inside and clear the drain instead.
Indoor fan runs, outdoor unit silent: look at the disconnect and breaker
When the blower runs but the condenser outside does nothing, the problem is between the panel and the outdoor unit. The condenser has its own two-pole breaker, typically 20 to 40 amps, and a weatherproof disconnect box on the wall beside it. Older disconnects in 1970s–90s subdivisions like Tyandaga and Millcroft often use pull-out cartridge fuses, and a blown cartridge looks normal from the outside; it takes a meter to tell.
A breaker that trips again within minutes is a different conversation; see why an AC trips the breaker. Also check the thin low-voltage wire that runs from the furnace to the condenser. String trimmers cut it in June, and mice chew it over winter when they nest in the base of the unit. A severed 24-volt wire means the contactor never gets the signal to pull in.
The contactor itself is a relay whose two contacts pit and burn over years of cycling. Ants are drawn into the enclosure and get crushed between the contacts, which sounds like a tall tale and is a genuine cause of a dead condenser. A worn contactor is a small part and one of the fixes in the $150–$350 band.
Outdoor unit hums or clicks but won't start: stop and call
A hum with no fan movement almost always means a failed run capacitor. The compressor and fan motor both need it to start, and when it weakens the motors just sit there drawing current and getting hot. Do not push the fan blade with a stick to get it going; your hand is beside a live fan, and running the unit on a dead capacitor leaves the compressor straining on locked-rotor current. Switch it off at the thermostat and call; the part and the price are covered in how much an AC capacitor costs.
Rapid clicking is usually the contactor chattering because the control voltage is low or its coil is failing. A loud buzz followed by silence, repeating every few minutes, can be the compressor going out on its internal overload. Cut the power at the breaker if it keeps trying; repeated locked-rotor starts do real damage.
What it costs to get an AC running again
- Diagnostic visit: $90–$150 and folded into the repair if you go ahead with it.
- Capacitor or contactor: $150–$350 including the part and the labour.
- Clogged condensate drain that tripped the float switch: cleared for $150–$350.
- Blown fuse or tripped breaker with no underlying fault: often just the diagnostic charge, plus an explanation of why it happened.
Most no-start calls are resolved in a single visit for $150–$450. The exception is a compressor that has failed electrically on an older unit; that is the moment to weigh repairing a 10-year-old AC against replacing it.
Why the first hot week is when this happens
A unit that has sat since September is being asked to start cold on the first 30°C afternoon, usually the same afternoon everyone else's is. Capacitors that lost capacity over the winter fail on that first hard start. Rodent damage, a cover left on, and a breaker someone switched off in the fall all surface at once. Running the AC for 10 minutes on a May day above 15°C, before you need it, catches most of this while a service slot is easy to get.
ZK Mechanical runs AC repair across Burlington, Oakville and Hamilton from our shop on North Service Road, at the same rate evenings and weekends. Tell us what the unit is doing when you book a visit so the likely parts are on the truck.