Capacitor Diagnostic · Free Tool

Capacitor MFD
Test Calculator

Enter your capacitor's label rating and your measured value to instantly diagnose whether it's failing. Works for run caps and dual run caps.

Capacitor Type

Voltage Rating

Label MFD Rating

Printed on the side of the capacitor

Measured MFD

Reading from your multimeter

HERM Label (compressor side)

HERM Measured

FAN Label (motor side)

FAN Measured

Capacitor Diagnosis—

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How to Test a Run Capacitor

Run capacitors fail more often than any other AC component. They store electrical energy that gives motors the boost needed to start and maintain rotation. As capacitors age, their microfarad (MFD) value drops, causing weak starts and eventual failure.

Safety First — Discharge the Capacitor

Before testing, discharge the capacitor. A charged capacitor can deliver a dangerous shock even after power is disconnected. Use an insulated screwdriver to short across the terminals (HERM-C and FAN-C separately for dual caps). Listen for a brief pop.

The 6% Rule

The industry standard tolerance is ±6% of the rated value. Any reading outside this range means replace the capacitor:

45 MFD label: acceptable range is 42.3 - 47.7 MFD
40 MFD label: acceptable range is 37.6 - 42.4 MFD
5 MFD label: acceptable range is 4.7 - 5.3 MFD

Common Causes of Failure

Heat exposure — most common cause. Caps rated for 105°C ambient last longer than 70°C caps
Voltage spikes — power surges damage internal dielectric
Age — even properly operating caps degrade 0.5-1% per year
Bulging or leaking — visible failure, replace immediately

Choosing a Replacement

Match the MFD rating exactly. The voltage rating must be equal to or greater than the original (440V can replace 370V). For dual run caps, both HERM and FAN values must match. Round versus oval shape doesn't affect performance — choose whichever fits your mounting bracket.

Frequently Asked Questions

How do I know if my capacitor is bad?

Common signs: outdoor unit hums but fan doesn't spin, AC won't start, capacitor is bulged/leaking oil, or measured MFD is more than 6% below rated. Capacitors degrade gradually — even small drops cause performance loss. Replace any cap reading >6% below label.

What's a dual run capacitor?

A dual run capacitor combines two capacitors in one case — one for the compressor (HERM, the larger value like 45 MFD) and one for the fan motor (FAN, smaller like 5 MFD). Both must test within tolerance independently. Common: 35+5, 40+5, 45+5, 55+5 MFD.

Can I replace a capacitor with a higher voltage rating?

Yes — voltage rating is the MAXIMUM voltage the cap can handle. A 440V capacitor can safely replace a 370V one (most modern caps are 440V anyway). The MFD value must match exactly — don't substitute different MFD values.

Are universal capacitors any good?

Universal/'super-cap' multi-tap caps work for emergency repairs but tend to fail faster than properly matched OEM-equivalent units. For long-term reliability, replace with the exact MFD/voltage spec from the data plate. Total cost difference is usually <$10.

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