As a general rule of thumb, a run capacitorwill far out last the same motor's start capacitor. A run capacitor will also fail or wear differently, making them slightly more involving when trying to determine if the time has come for replacement. Start capacitors will commonly fail catastrophically, making the decision on when to replace obvious, limiting time spent troubleshooting.
When a run capacitor begins to perform outside the allowable range, it is most often indicated by a dropping of the rated capacitance value (the microfarad value has gone down). For most standard motors, a run capacitor will have a "tolerance" specified describing how close to the rated capacitance value that the actual value may be. This will be usually +/- 5% to 10%. For most motors, as long as the actual value is is within the 10% mark of the rated value, you're in good shape. As a run capacitor is used, and the capacitance drops outside of this range, you’ll need to look at a replacement.
In some cases, due to a defect in a capacitor's construction or sometimes caused by a non capacitor related motor issue, a run capacitor will bulge from internal pressure. For most modern run capacitor designs, this will open the circuit, dis-connecting the internal spiral membrane, as a protective measure to prevent explosion.
The test in this case is simple; if its bulging, time to replace. If you measure no continuity across the terminals, it is also time to replace.
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