Your AC is running, you can hear the outdoor unit, but indoor temperatures are not dropping. This is the single most common AC service call we receive between May and September across the GTA. There are 14 distinct causes, ranging from a $0 thermostat reset to a $3,500 compressor replacement. This guide walks through each one in the order our techs check them, with diagnostic steps you can run yourself before calling.
Quick triage: what to check first (5 minutes)
- Thermostat set to COOL mode (not OFF or HEAT) and setpoint at least 3°C below current room temperature.
- Thermostat batteries fresh (smart thermostats have low-battery warnings; older models do not).
- Air filter clean. A clogged filter blocks airflow and is the #1 cause of "AC not cooling" calls. Replace if visibly grey.
- All supply registers open and unblocked by furniture or rugs.
- Outdoor condenser unit clear of debris (leaves, grass, mulch). Fins should be visible.
- AC breaker not tripped. Walk to your electrical panel and check.
If everything above checks out and the AC still is not cooling, the cause is mechanical. Continue through the diagnostic list below.
The 14 most common causes (in order of frequency)
Clogged air filter
Restricts airflow across the indoor evaporator coil, which causes the coil to freeze, which stops cooling. Symptom: visibly dirty filter, weak airflow at registers, possibly ice visible on refrigerant lines. Fix: replace filter ($15 to $30). If the coil is already frozen, turn the AC off and the fan ON for 60 minutes to thaw before running again.
Frozen evaporator coil
The indoor coil ices over due to restricted airflow, low refrigerant, or a failing blower motor. Symptom: ice visible on refrigerant lines at the indoor air handler, water leak at the indoor unit when ice melts, AC blowing room-temperature air. Fix: turn AC off, fan ON for 60 to 90 minutes to thaw, then diagnose the underlying cause. Repair cost depends on cause: $0 (filter) to $1,400 (coil replacement).
Low refrigerant from a leak
Refrigerant does not get used up; if levels are low there is a leak. Symptom: AC works but slowly, longer run times, escalating electricity bill, ice on refrigerant lines, hissing or bubbling sounds. Fix: TSSA-certified tech must find and repair the leak before recharging. Leak repair plus recharge runs $485 to $1,200 depending on leak location.
Failed capacitor (start or run)
The capacitor stores the electrical jolt needed to start the compressor and fan motors. When it fails, you often hear the outdoor unit humming but the fan does not spin. Symptom: outdoor unit humming, fan blade can be flicked into motion with a stick (do not do this), AC turns on briefly then shuts off. Fix: capacitor replacement runs $185 to $285 installed. Same-day repair on every truck.
Failed contactor
The contactor is the electrical switch that connects the outdoor unit to power when the thermostat calls for cooling. Symptom: outdoor unit completely silent despite thermostat calling for cool, or constantly running. Fix: contactor replacement runs $165 to $245 installed.
Condenser fan motor failure
The outdoor fan motor pulls air across the condenser coil to release heat from the refrigerant. When it fails, the system overheats and shuts down on high-pressure safety. Symptom: outdoor fan not spinning while compressor runs, compressor cycles on and off rapidly. Fix: condenser fan motor replacement runs $385 to $585 installed.
Blower motor failure (indoor)
The blower motor moves cooled air from the indoor coil through your ductwork. Failure causes airflow to drop, the coil freezes, and cooling stops. Symptom: weak or no airflow at any register, indoor coil frosting. Fix: ECM blower motor replacement runs $485 to $785 installed.
Dirty outdoor condenser coil
Cottonwood seed, grass clippings, and lawn debris coat the outdoor coil fins and block airflow. Symptom: outdoor unit running hot to the touch, longer cycles, weaker cooling on hot days. Fix: professional coil cleaning runs $245 to $385.
Thermostat failure or miscalibration
Thermostat reads inaccurate room temperature, fails to send the cool signal, or has a stuck relay. Symptom: thermostat display shows wrong temperature, AC runs constantly or never turns on, smart thermostat shows no connection. Fix: thermostat replacement runs $185 to $485 depending on smart vs basic model.
Restricted or leaky ductwork
Disconnected ducts, crushed flex duct, or major air leaks dump cool air into attics, basements, or wall cavities instead of living spaces. Symptom: certain rooms never cool, ductwork visible damage in basement or attic. Fix: duct repair runs $250 to $1,500 depending on extent.
Undersized AC for the home
The unit was sized too small at installation, often by going off square footage alone instead of a Manual J load calculation. Symptom: AC runs constantly on hot days but cannot reach setpoint, even when everything else checks out. Fix: replace with correctly-sized unit ($3,500 to $7,500). Read our <a href="/blog/ac-sizing-guide-mississauga">AC sizing guide</a> for the math.
Air handler short-cycling
AC turns on, runs briefly, then shuts off without cooling the home. Caused by oversized system, refrigerant overcharge, electrical issue, or failing pressure switch. Fix depends on cause: $150 to $850.
Failing reversing valve (heat pump only)
On heat pump systems used for cooling, the reversing valve directs refrigerant flow. A stuck or failing valve can leave the system in heating mode while the thermostat calls for cool. Symptom: heat pump blowing warm air despite cool mode setting. Fix: reversing valve replacement runs $650 to $1,400 installed.
Compressor failure
The most expensive and least common cause. The compressor is the heart of the AC. Symptom: outdoor unit silent or grinding, no cooling at all, electrical breaker trips when AC starts. Fix: compressor replacement runs $1,800 to $3,500. On units 10+ years old, full AC replacement usually wins on lifetime cost.
DIY vs call-the-tech decision tree
Use this rule of thumb to decide what to handle yourself versus when to call a TSSA-certified technician:
| Task | DIY safe? | Why |
|---|---|---|
| Change air filter | Yes | Standard homeowner maintenance. |
| Reset breaker once | Yes | One reset is safe. Repeated tripping means stop and call. |
| Clear debris from outdoor coil | Yes | Use a garden hose on gentle setting. Cut power first. |
| Calibrate thermostat batteries | Yes | Standard battery swap. |
| Flush condensate drain | Borderline | A cup of vinegar in the access port is safe. Snaking the drain is professional work. |
| Refrigerant work (any kind) | No | Illegal in Ontario without TSSA certification. Refrigerant handling requires specific equipment and license. |
| Capacitor replacement | No | Capacitors hold lethal voltage even after power is cut. Discharge requires proper tools. |
| Gas valve, ignition, electrical inside unit | No | TSSA gas-fitting license required by law. |
| Compressor or coil replacement | No | Refrigerant recovery, brazing, evacuation, recharge all require certification. |
When AC repair turns into AC replacement
Use this simple rule when facing a major repair quote: multiply the AC age in years by the repair cost in dollars. If the result exceeds $5,000, replacement usually wins on lifetime cost.
- 6-year-old AC + $400 capacitor: 6 × 400 = 2,400 → repair.
- 10-year-old AC + $1,400 coil: 10 × 1,400 = 14,000 → replace.
- 12-year-old AC + $850 fan motor: 12 × 850 = 10,200 → replace (or get a hybrid heat pump for double the rebates).
- 14-year-old AC + $2,500 compressor: 14 × 2,500 = 35,000 → replace immediately, ideally with a heat pump.
If you are replacing, consider a cold-climate heat pump instead of a standard AC. Heat pumps qualify for up to $10,000 in federal and Enbridge rebates and provide both cooling and heating. Read our <a href="/heat-pump-vs-gas-furnace-ontario">heat pump vs gas furnace comparison</a> for the 12-year cost math.
Prevention: stop the most common AC failures before they happen
- Change the air filter every 30 to 60 days during cooling season (more often if you have pets).
- Annual professional tune-up in spring catches 70 percent of summer failures before they happen. EcoFrost AC tune-up runs $149 flat across the GTA.
- Keep 2 feet of clearance around the outdoor condenser unit. No mulch, shrubs, or lawn furniture against the coil.
- Rinse the outdoor coil with a garden hose once a month during cottonwood season (typically June in Mississauga).
- Flush the condensate drain with a cup of distilled vinegar every spring to prevent algae blockages.
- Run the system in fan mode for 5 minutes after each cooling cycle to dry out the indoor coil and reduce mould risk.
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