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A central air conditioner that hums but does not start, an exhaust fan that runs slowly, or a furnace blower that takes several seconds to get up to speed are all common signs of a failing motor capacitor. Capacitors are among the most frequently replaced components in HVAC and motor-driven appliances, and diagnosing a bad one correctly saves a service call and helps you understand what needs replacing before an electrician or HVAC technician arrives.
A motor capacitor stores and releases electrical energy to give a single-phase motor the phase shift it needs to start and run. Start capacitors provide a large burst of energy to get the motor spinning; run capacitors stay in the circuit continuously to improve motor efficiency and torque. Both types fail over time — most commonly from heat, age, and voltage spikes — and a failed capacitor typically causes the motor to fail to start, run weakly, or burn out from the sustained starting current it draws while stuck.
Recognizing the symptoms before testing saves time and narrows the diagnosis before you open anything up:
Capacitors store electrical charge even after power is disconnected. A large HVAC run capacitor (370V or 440V rating) can hold enough charge to deliver a painful and potentially dangerous shock if you touch the terminals immediately after shutting down the system. Before testing or handling any capacitor, turn off power at the disconnect or breaker, wait at least 5 minutes, then discharge the capacitor through a 20,000-ohm, 5-watt resistor across the terminals — or use an insulated screwdriver to bridge the terminals briefly (sparks are normal and expected). Do not short the terminals with a bare wire — the current surge can damage the capacitor’s internal dielectric. Never handle capacitors with both hands simultaneously.
In Ontario, work on HVAC electrical components beyond visual inspection typically requires a licensed electrician or HVAC technician. The Ontario Electrical Safety Code (OESC) and the Electrical Safety Authority (ESA) classify motor and capacitor replacement in residential HVAC equipment as electrical work requiring an ESA permit if it involves modifying the circuit. Replacing a capacitor on the same terminals with the same-rated part is generally considered maintenance, but consult the ESA homeowner guide if you are unsure whether your situation requires a permit.
A digital multimeter with a capacitance measurement function (marked with the letter C or a capacitor symbol) can test a capacitor directly. Set the meter to the capacitance range that covers the capacitor’s rated value (printed on the label in microfarads, µF). Discharge the capacitor as described above, then touch the meter probes to the terminals. The reading should be within 6% of the rated value printed on the label. A reading more than 6% below the rated value means the capacitor is weak — replace it. A reading of zero or OL (overload) means the capacitor is open (failed completely). A reading near zero that rises briefly then returns to zero is a shorted capacitor.
For dual-run capacitors (a single can with three terminals labelled C, HERM, and FAN), test each pair of terminals: C to HERM for the compressor winding, and C to FAN for the fan motor winding. Either side can fail independently — a dual capacitor with one good side and one bad side should be replaced as a complete unit. See our guide on how to check continuity on a switch or wire for guidance on multimeter basics that apply directly to capacitor testing.
If your multimeter does not have a capacitance function, use the resistance (ohms) setting as a rough check. Discharge the capacitor, then touch the probes to the terminals. A good capacitor will show a brief rise in resistance reading (as the meter’s test current charges the cap) and then climb toward infinity — this indicates the capacitor can hold charge. A capacitor that reads near zero ohms immediately is shorted. A capacitor that shows no movement at all (immediate OL from both probes) is open. The ohms test cannot confirm the capacitor meets its rated µF value — it only tells you whether it is shorted or open. A capacitor that passes the ohms test but fails to start the motor may be weak and still needs replacement.
The label on the capacitor shows the capacitance value (µF), the voltage rating (VAC), and the type (start or run). The voltage rating must be equal to or higher than the voltage in your application — you can replace a 370V run capacitor with a 440V capacitor, but not the reverse. The µF value must match the original specification — upsizing or downsizing the capacitance changes motor behaviour and can cause overheating or reduced torque. If the original label is unreadable, look up the motor’s model number to find the OEM capacitor specification. Using a generic-value capacitor outside the original specification voids the motor warranty and can accelerate failure.
Capacitor failure shows up most commonly in three applications in Ontario homes:
The Electrical Safety Authority (ESA) notes that DIY HVAC electrical work is a common source of residential electrical incidents in Ontario. If you are not confident diagnosing or replacing a capacitor safely, call a licensed HVAC technician or electrician. A capacitor replacement is typically a 15-minute job for a professional and costs significantly less than the motor damage caused by running a motor continuously against a failed capacitor.
The most common mistake we see when homeowners diagnose a bad capacitor is testing the capacitor while it is still in the circuit — reading the capacitance while the capacitor is wired to the motor windings gives an incorrect result because the motor windings add inductance that affects the reading. Always disconnect the capacitor from the circuit before testing. The second common mistake is replacing a run capacitor with a start capacitor or vice versa because the µF values appear similar — start capacitors are rated for intermittent duty and will burn out rapidly if left in a run circuit. Check the label: start capacitors are typically 50–400 µF and run capacitors are 2–60 µF, but the best confirmation is the type printed on the can. If you find a capacitor with a visible defect (bulged top, oil residue), do not test it — replace it directly and bring the old unit to a battery recycler, as capacitors contain electrolytes that should not go to landfill.
Suspect a bad capacitor in your HVAC system or motor? Our ESA-certified electricians serve the GTA. Call us at 416-838-9006 or visit our contact page — we will get back to you the same day.
Electrician Since 2008 Journeyman Electrician Designated Master Electrician at EZSMART Corp
