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AC won't turn on at all

An AC that won't start is often the easiest kind to fix — most of the time it's a switch, a breaker, or a safety device doing its job, not a dead machine.

tuneWhat are you noticing?
Select anything that matches and the checklist below reorders to fit. Not sure? Skip it.
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Start here

Start at the thermostat — the cheapest check on this list. Make sure it's set to COOL, the setpoint is a few degrees below the room temperature, and the screen isn't blank or fading. If the display is dead, put in fresh batteries. Two minutes, free, and the most common cause to rule out first.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order
Beginner-friendly Doable, with care Safety-critical Call a pro

visibilityHow to check

  1. Check the screen: blank or fading digits point to dead batteries (or, if it has no batteries, to the low-voltage fuse cause further down).
  2. Check the settings: mode on COOL (not OFF or HEAT) and the setpoint at least 2–3°C below the current room temperature, then wait up to five minutes — many thermostats have a built-in restart delay to protect the compressor.
buildNo tools schedule2 min

handymanHow to fix it

Set the mode to COOL, drop the setpoint below room temperature, and replace the batteries if the display is weak. Then give the system five minutes — the compressor restart delay is normal, not a fault.

  1. Pull the thermostat face off its wall plate (most hinge or slide off) and swap in fresh batteries — usually AA or AAA, printed inside the cover.
  2. Set mode to COOL and the setpoint 2–3°C below the room temperature so the system gets a clear call for cooling.
  3. Wait five minutes. If the indoor fan starts and the outdoor unit follows, this was the whole fix.
schedule5 min savings$0–10
Thermostat batteriesUsually AA or AAA — the size is printed inside the thermostat cover

visibilityHow to check

  1. Open the electrical panel and find the AC breakers — often two, labelled something like "AC", "condenser", or "furnace/air handler". A tripped handle sits in the middle, between ON and OFF.
  2. Outside, look at the grey disconnect box mounted on the wall beside the condenser. Make sure its pull-out block is present, pushed in fully, and (if it has one) the switch is ON.
  3. If someone serviced or painted near the unit recently, a pulled disconnect is a classic cause of this exact symptom.
buildNo tools schedule5 min

handymanHow to fix it

warning One reset. That's the rule.Resetting a breaker is a normal homeowner task — once. Stand to the side of the panel, use dry hands, and flip it with one firm motion. If it trips again immediately, the breaker is protecting you from a real fault; every additional reset pushes fault current through the circuit again. Leave it off and bring in a licensed electrician or HVAC technician.

Reset a tripped breaker once: push it firmly to full OFF, then back to ON. Reseat the outdoor disconnect block fully. If the breaker trips again right away, stop — that's a fault, not bad luck, and it's the escalation on this page.

  1. With dry hands, push the tripped breaker firmly to full OFF first — a tripped breaker won't reset from the middle position — then flip it to ON.
  2. Outside, push the disconnect block in until it seats flush.
  3. Give the system five minutes to restart. If the breaker holds, watch for a repeat over the next few days; a breaker that trips occasionally under load still deserves a professional look.
  4. If it trips again immediately: leave it OFF, pull the disconnect back out, and call a licensed electrician or HVAC technician. Do not keep resetting it.
schedule5 min savingsusually free

visibilityHow to check

  1. Find the indoor unit (furnace or air handler — often in the attic or a closet on newer builds) and look underneath and beside it for a shallow drain pan.
  2. Standing water in the pan, or water stains around it, points squarely here. The float switch is a small cylinder or box riding in the pan or clipped into the drain pipe.
  3. Follow the white PVC drain line to where it exits the house. No drip outside on a humid day while the pan sits full is the classic clogged-line signature.
buildflashlight schedule10 min

handymanHow to fix it

warning Attic units: watch your step, respect the waterIf the air handler is in the attic, step only on the joists or laid boards — the drywall between them won't hold you — and mind that a full pan means water has already been close to your ceiling. Bail it gently rather than tipping it. If the unit is buried in a tight attic or the pan has already overflowed, this is a quick call for an HVAC company instead.

Clear the clogged condensate line — a wet/dry vacuum sealed onto the line's outdoor end for a couple of minutes usually pulls the clog through — then empty the pan. The float drops and the AC comes back on its own.

  1. Find where the condensate line exits the house (a small white PVC pipe near the outdoor unit or a wall). Seal a wet/dry vacuum hose over the end — a rag wrapped around the joint works — and run it for two to three minutes.
  2. Indoors, bail or vacuum the standing water out of the drain pan so the float switch can drop back down.
  3. If the line has a capped tee near the indoor unit, pour a cup of warm water down it and watch it run out the far end freely.
  4. Run the AC and check the outdoor end of the line the next humid day — a steady drip means the drain is doing its job again. A clog that keeps returning is worth a pro visit to re-slope or flush the line properly.
schedule30 min savings$0–25

verifiedWhy this one goes to a pro

Listening from outside the unit is as far as this one goes. The contactor sits in the condenser's electrical compartment with 240-volt line terminals that stay live even when the thermostat is off, next to a capacitor that holds a charge after power is cut. Replacing it means working at those lugs — in every province that's a job for a licensed technician, and it's a routine, inexpensive one. Your part is done: tell the tech "indoor fan runs, outdoor unit silent" and you've saved them (and yourself) diagnostic time.

gavel Check your province first AB: permit + inspection required · BC: permit + inspection required · ON: permit + inspection required · PE: permit + inspection required · QC: licensed professionals only
Conservative default pending review: treat line-voltage work inside the condenser as requiring permit/inspection. REVIEW: confirm this province's homeowner-electrical rules before publish.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

verifiedWhy this one goes to a pro

Compressor and board diagnosis means live line-voltage testing and, on the compressor side, opening a sealed refrigerant system — work that federal regulations reserve for certified technicians everywhere in Canada. There is no homeowner version of this repair. What you can do is valuable anyway: bring the tech your ruled-out list from this page, and ask for the diagnosis in writing before approving a compressor-sized bill — on an older unit it's worth comparing against replacement.

gavel Check your province first AB: licensed professionals only · BC: licensed professionals only · ON: licensed professionals only · PE: licensed professionals only · QC: licensed professionals only
Any work opening the refrigerant circuit is restricted to certified technicians across Canada under the federal halocarbon regulations (SOR/2016-137); line-voltage board work is additionally restricted provincially. REVIEW: confirm per-province wording.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

visibilityHow to check

report Power off to open, and know why the meter step is differentKill power to the indoor unit before opening the panel and handling the fuse — that part is safe and simple. The multimeter reading is different: it needs the power back on, with your hands near a live board that also carries 120/240 V line voltage a few centimetres away. Keep your probes on the low-voltage terminal strip only, one hand behind your back if you're nervous, and if that step doesn't sit right with you, stop at the visual fuse check — confirming the transformer is a fast, inexpensive job for an HVAC tech.
  1. Turn off power to the indoor unit at its switch or breaker, then open the blower compartment door to expose the control board.
  2. Find the automotive-style blade fuse on the board (usually purple 3 A or tan 5 A). Pull it and look through the plastic: a broken or blackened strip means it's blown.
  3. If the fuse looks fine, the transformer is next in line: with power restored and the door switch held per the gate below, a multimeter set to AC volts should read about 24–28 V across the R and C terminals. No voltage with a good fuse points to the transformer.
  4. Either way, note what you found — if a new fuse blows again, the real fault is a short in the thermostat wiring, and that diagnosis is a pro's job.
buildscrewdriverbuildmultimeter schedule20 min

handymanHow to fix it

Replace the blade fuse with one of the exact same amp rating (never bigger) with the power off. If the new fuse blows too, there's a short in the 24-volt wiring — stop there and book a pro; a failed transformer is also a pro-level swap.

  1. With power to the indoor unit off, pull the blown fuse straight out of its holder on the control board.
  2. Press in a new fuse with the identical amp rating printed on the old one — a bigger fuse defeats the protection and can cook the transformer or wiring.
  3. Close the door, restore power, and set the thermostat to COOL. A working display and a starting system means the fuse was the whole story.
  4. If the new fuse blows immediately or within days, there's a short — often a nicked thermostat wire. Leave the power off and have an HVAC tech trace it.
schedule15 min savings$2–10
Low-voltage blade fuse (3 A or 5 A)Match the amp rating printed on the blown fuse — never go bigger
sort

Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.