A run capacitor on an outdoor condenser fan or compressor is one of the cheapest parts in the system, and one of the easiest to get wrong. The can carries the information you need. Before you order anything, read the printed microfarad value (usually marked MFD or µF), the voltage rating, and the tolerance if it is printed. Those numbers, along with the motor nameplate and the condenser's model data, are what a replacement has to satisfy.
What the numbers mean
The microfarad value sets the phase shift the capacitor provides to the motor winding. Install a value far from the label and the motor may start sluggish, run hot, or fail to start. Small differences within the tolerance band are normal. Voltage works differently: a replacement must be rated at or above the voltage the circuit sees. Going higher on voltage is a safe margin, while going lower is not. Many run capacitors are dual rated, with a common terminal, a fan terminal, and a compressor terminal, and the labels show which terminal goes where. Do not assume wire colors tell you the function. Trust the terminal markings on the can and the wiring diagram on the unit.
Test before you pull
A multimeter with a capacitance function gives you a reading to compare against the label. Shut the system off at the disconnect, not just at the thermostat. Discharge the capacitor with a resistor-type discharge tool rather than a bare screwdriver across the terminals, then confirm with the meter that the voltage is near zero before you touch anything. Capacitors can hold a charge after the power is off. Disconnect the wires, take the reading on the correct terminals, and write the number next to the label value. A reading well below the label, or a can that is bulging, leaking, or has a swollen top, is the usual reason to replace it. If you are not comfortable working inside an energized panel, stop here and call a licensed technician.
Where a monitor fits
Remote monitoring is getting easier to buy. ACHR News reported on the Trane and American Standard HVAC performance monitoring kit, which gives dealers remote access to system performance data and diagnostics across virtually any 24V system. Dealer-facing data is useful, but it does not replace the label on the capacitor. A monitor can flag a symptom, while the meter and the can tell you whether that part is the cause. Keep the capacitor's ratings, the date of the swap, and your reading with the unit's service record, so the next person who works on the system does not start from memory.
Ratings are test conditions
ACHR News also covered Q-PAC's Florida laboratory earning AMCA 270 accreditation for fan array testing, a reminder that performance numbers come from a defined test setup. The capacitor label works the same way. The microfarad and voltage values describe the part as built, and your meter reading describes it today. Compare the two before you order, and keep the label in view while you do. A part that matches the can and reads close to its rating is a good sign. A part that reads far from the can has told you what it needs, and the label tells you what to buy.