For the record, "condenser" is an age old term, and "capacitor" is a newer term for the same device. Automotive ignition capacitors are still commonly called condensers.

Many people will replace a condenser with regular periodic maintenance, just because they have no way to tell how much longer it might last. But some new condensers might be bad right out of the box, or might fail very shortly after installation. My approach is to carry a known good condenser in the traveling tool kit, and don't replace the old one until it fails. For this to work you have to be prepared to change one at some inconvenient time, possibly on the side of the road, but you should be prepared for such a possibility at all times regardless.

For decades I have had very good luck with condensers, but in recent years there have been lots of reports of condensers that fail prematurely. In December 2013 I had one fail after 18 months and 9000 miles. The two months later another one failed after only 257 miles. Even regular replacement at reasonable intervals could not avoid these premature failures. This begs the question, how do you test a condenser to determine if it is good or bad? Well, even when you can test a condenser, this is still not the final solution, as a condenser might test good one day and fail the next day. But periodic testing of the condenser in the car might (sometimes) disclose a deteriorating condition before it actually fails. Testing can also verify a suspect condenser that might fail soon after installation. If testing reveals an apparently good condenser, then you can look elsewhere for a problem without having to replace the part.

For anywhere from $20 to $200 you could buy a condenser tester. The real tester can apply a high voltage (500 to 600 volts) to test for leakage, and can also apply an AC current to actually measure the capacitance (storage capacity) of the device. These two tests more closely stress the capacitor in the same way actual operation does. But considering that a condenser is a cheap part, and you should always carry a known good spare anyway, the condenser tester seems a bit overboard for the average shade tree mechanic. Fortunately there is a way to do a rudimentary test with a common analog (moving needle) ohm meter.

1.) Remove the condenser from the engine (or at least disconnect the lead wire). Note the small metal connector located on the end of the condenser. This connector is the "hot" or power connection. The metal case of the condenser is the grounding point. Discharge the condenser by shorting the lead wire to the case.

2.) Switch the meter to the ohms position. Place the red lead into the "ohm" connector on the meter. Insert the black lead into the "com" or common connector on the meter. Set the resistance range to the highest available setting (if it is selectable). Connect the test leads together and zero the meter. If the meter won't zero replace the battery. (Yes, an ohm meter has a battery).

3.) Touch the red lead to the hot connector on the condenser. Place the black lead to the metal case on the condenser. The meter's needle should jump slightly to the right (toward 0-ohms), then should drop back to the left towards infinite resistance). Hold the leads in place for 15 to 20 seconds. This action places charge in the condenser. If the test shows any reading other than infinity, the condenser is leaking and needs to be replaced.

4.) Remove the leads and reverse the placement to the condenser. Move the red lead from the hot connector to the metal case, and move the black lead from the metal case to the hot connector. At the moment where both leads are touching the correct points, the meter should jump towards the right. The second time the needle may move twice as far, as this action discharges the condenser. Holding the leads in contact should again result in movement of the needle back toward infinite resistance.

5.) Movement of the meter's needle indicates the condenser is good. If no movement was indicated on the meter in any circumstance, the condenser is bad and must be replaced. Retest the condenser several times for a consistent reading.

In operation the condenser will "ring" at up to 300 volts, so the condenser needs to be rated much higher, no less than 600 volts DC. The ignition will likely work with any capacitance value between 0.05 and 0.6 microfarad. Too high or too low value may eventually transfer metal from one side of the contact points to the other side leaving a pit and a point. Capacity of spark coil capacitors ranges from 0.2 microfarad to 0.33 microfarad. Almost all automotive coils use a 0.25-0.29 microfarad capacitor.

A capacitor may absorb moisture over a long period of time, and moisture can cause failure of the condenser. So it is possible that a condenser stored for 10 years or so might be bad or might fail prematurely in service. It is a good idea to check your traveling spare condenser occasionally.